If we’ve learned anything about postseason baseball over the last five years or so, it’s that relievers become ever more important when the lights get the brightest. Don’t get us wrong, exactly; of course starters still matter, and of course you’ll immediately think of the 2019 Nationals, who went with the “what if we make the entire staff out of starters” strategy, to fine results.

But that group of Nationals seems more like an outlier than the plan these days. With more days off between games, no tomorrow should you lose said games and a greater understanding of what letting non-elite starters turn over a lineup three or four times actually means, managers have become more and more aggressive about going to the bullpen early in October, to the point that it’s become more an expectation than a surprise.

The point is that you’re much more likely to see 2016 Andrew Miller (who faced 73 batters in 10 appearances over 15 Cleveland playoff games) than you are 2016 Zack Britton (who waited for a save opportunity that would never come as six other relievers entered in the AL Wild Card Game).

Not that you don’t already know this just from having watched the games, but to show it in graph form …

After many years of relievers throwing about one-third of postseason innings, their share began climbing in 2015 and topped 50% for the first time ever in 2020. (We’ll stipulate that the 2020 playoffs were unusual, to say the least, but 2018 was only a fraction of a percent behind it, and the Nationals singlehandedly affected 2019.)

So, as we head full speed into what ought to be quite an entertaining month of baseball, we bring to you the 2021 postseason reliever tiers, highlighting the various groups of relievers you’ll expect to see and need to know as the playoffs move on.

We thought about trying to add some science to this, attempting to predict or project 2021’s best October relievers, and then we remembered how playoff baseball tends to go, and so we thought we’d better group them together in a semi-scientific-but-mostly-not way to come out with approximately two dozen important relievers, in tiered groups. This is not a list of “lowest ERA, ordered.” It is not a list of every reliever that will appear in October, lest we go 75 deep. It's a list of the arms we think will be most important. (Note: We are including some pitchers below whose teams have not yet clinched, but will end up being important if they do.)

With apologies to Pete Fairbanks, Luis Garcia, Garrett Crochet, Ryan Tepera, Camilo Doval, Joe Kelly, Adam Cimber, José Álvarez, Phil Bickford, Michael Kopech, J.P. Feyereisen, Drew Steckenrider and whichever other reliever you’re mad we omitted …

Tier 1: The two kings

These are the top dogs, the Main Dudes. These are not out of nowhere pop-up types or the fading big-name veterans; these two were each dominant in 2019 and '20, and have continued to be so in 2021. They've been great. They're still great.

Liam Hendriks, RHP, CWS
Josh Hader, LHP, MIL

Hendriks entered the season as baseball’s best reliever, and the White Sox paid him accordingly, giving him four years and $54 million to come to Chicago. One hundred and eight strikeouts later -- against only seven walks, giving him one of history’s best K/BB rates -- not much has changed with his elite status.

Hader, somehow, has managed to improve even beyond the historical dominance he showed in his first four seasons with Milwaukee, posting a 1.27 ERA thanks in large to part to allowing as many homers (3) in 56 2/3 innings as he did last year in 19 innings. Not bad, paired with the highest strikeout rate this side of Jacob deGrom.

Tier 2: The high-end 30-somethings

These four have had wildly different career paths, with several needing to find a new home before they found success. One is one of the better closers of his generation; one is his setup man; another wasn't the closer as recently as six months ago. (This is the tier where Devin Williams would have appeared before his broken hand put him out for most or all of the postseason. Since he's not included, we get to title it 30-somethings.)

Kenley Jansen, RHP, LAD
Blake Treinen, RHP, LAD
Ryan Pressly, RHP, HOU
Giovanny Gallegos, RHP, STL

Every time we think Jansen has seen his best days, he surprises us -- and he certainly looked good striking out the side to finish off the Padres on Wednesday night. After a rough July, he's been very good over the last two months (1.32 ERA and a 37/8 K/BB since Aug. 1), and he's regained some of the velocity that had been declining in previous years. But Jansen, 34, also has his highest walk rate (13%) since he was a rookie back in 2011, and thanks to the crime of a reliever being very good and not perfect, it just never feels like Dodger fans trust him as much as you'd think his track record should allow.

His running mate in Los Angeles, Treinen, is having a pretty special season of his own, completing a turnaround from “historic 2018” (0.78 ERA) to “inconsistent 2019-20” (4.59 ERA) to this year, when he’s posted a 2.02 ERA and upped his strikeout rate from 21% to 30%. He's doing it in an interesting way, too, since after years using his sinking fastball as his primary pitch, he's now using his cutter and slider in ways he's never done before.

Pressly is now in his fourth season in Houston since the Astros swiped him from the Twins, and he just keeps on dominating. Prioritizing his slider over his curve, Pressly has 80 strikeouts against only 12 walks this year, and a wild 213/34 over his Astros career. This year, he's got a career-low 5% walk rate.

You might not consider Gallegos a big name, but we do. While St. Louis fans rightfully bemoaned the loss of Luke Voit as he slugged in the Bronx, the trade did return Gallegos, who is now into his third year of excellent relief work, posting a 2.78 ERA and more than five times as many walks as strikeouts as a Cardinal. His ascension to the closer’s role, replacing All-Star Alex Reyes, is a big part of the Cardinals’ wild run to the playoffs.

Tier 3: The big names with question marks

These names, to be honest, carry as much or more star power than some in the group above it, and if we’d done this exercise on or around June 15, we’d have been including half of these stars in our first group. But for each of them, be it second-half struggles or some specific flaw in their game, there’s an added level of uncertainty that comes with putting them in a big spot right now -- and if there’s anything managers hate, it’s bullpen uncertainty.

(In Reyes’ case, he’s had such a rough go of it -- a 6.08 ERA over the last two months -- that we’re not sure he’ll have any meaningful role at all unless the Cardinals advance beyond the NL Wild Card Game.)

Aroldis Chapman, LHP, NYY
Craig Kimbrel, RHP, CWS
Matt Barnes, RHP, BOS
Will Smith, LHP, ATL

Chapman has had one of the most wild seasons we can remember. In his first 23 appearances, he allowed one single earned run; in his next 12 appearances, he allowed 14 earned runs. It’s been better since then, as a 2.19 second-half ERA will attest to, but the walk rate remains high, and his appearances in high-pressure spots can charitably be described as “uncomfortable watching.” Still, few relievers in history have such a long track record of success as he does. The Yankees haven't clinched yet, but they're in the driver's seat for an AL Wild Card spot.

Kimbrel, like Chapman, got off to a fantastic start this year, which is why the White Sox paid such a high price to acquire him from the Cubs on July 30. But it hasn’t been such smooth sailing on the South Side, where he’s allowed a .326/.475 OBP/SLG after .190/.146 marks with the Cubs. The Sox had the division wrapped up long before he arrived, so this acquisition was entirely about what he can do in October, which means that he's going to have to pull it together, and soon.

Barnes -- whose Red Sox have not yet clinched -- could probably have the same description as the two names above him: Track record of success, wonderful start to 2021, hugely problematic stretches since. It’s been so troublesome -- in a seven-game span in August, he allowed nine earned runs -- that he’s lately been pitching in the sixth inning of losses during Boston’s most important stretch of the season.

In Atlanta, Smith is the biggest name in a generally underwhelming Braves bullpen, and for good reason, because he’s been one of baseball's better relievers ever since joining the Brewers in 2014. But his first season with the Braves, 2020, was a rough one; he was delayed by COVID and ended up throwing just 16 innings. His 2021 has been better, but it seems like clean innings are a rarity. There are pitchers on this list further down that we like better, but none as important to their teams as Smith will be.

Tier 4: The under-the-radar arms you don’t know enough about

Your mileage may vary on “under the radar,” of course. If you’re a serious fantasy player or a fan of one of their teams, then of course you know these guys. But for the larger population of general baseball fans, these aren’t necessarily names that jump off the page, or in some cases, ring a bell for you at all. Don’t however, let that relative anonymity prevent you from realizing just how good these arms have been.

Yes, the Yankees have three of them. And yes, this tier includes players on a few teams (Yankees, Mariners, Blue Jays, Red Sox) that have not clinched. Half of those four teams will make it, the other half will not.

Clay Holmes, RHP, NYY
Michael King, RHP, NYY
Jonathan Loáisiga, RHP, NYY
Alex Vesia, LHP, LAD
Paul Sewald, RHP, SEA
Garrett Whitlock, RHP, BOS
Tyler Rogers, RHP, SF
Jordan Romano, RHP, TOR

Ten months ago, Holmes was non-tendered by Pittsburgh before coming back on a Minor League deal, and why not, given that he had a career 5.91 ERA and had pitched just once in 2020. Two months ago, he was traded to the Yankees as an elite ground-baller with trouble throwing strikes. Since then? He’s got 31 strikeouts against only three walks, allowing just four runs, in 25 innings -- and he’s become one of Aaron Boone’s most trusted relievers.

Fellow righty King made the Opening Day roster as a multi-inning reliever, and immediately impressed by retiring 16 straight in a six-inning relief appearance on April 4. But after being pressed into the rotation in July (5.24 ERA in five mostly unimpressive starts) and then missing two months due to a finger injury, he was off the bullpen radar until returning three weeks ago ... and looking fantastic. In six outings, he's allowed two earned runs while striking out 13. The Yankees' bullpen looks a lot different than it did just two months ago.

Speaking of which, Loáisiga had been kicking around as a swingman for three years, showing flashes but also inconsistency, before emerging in 2021 as one of the better relievers in baseball (2.22 ERA) upon moving away from the four-seamer that never really worked and toward the 98 mph sinker that does. After missing a few weeks with a sore shoulder, he returned on Wednesday to touch 99 mph, giving confidence he's healthy.

Dodger rookie Vesia didn’t gain much notice allowing 10 runs in five games for Miami last year, and Dylan Floro was the big name in the winter trade that sent Vesia to Los Angeles. Score another one for the Dodger pitching machine; fueled by a fastball with elite rising action, Vesia has become a high-leverage lefty, particularly since the Trade Deadline, as he’s allowed two earned runs in 22 outings, whiffing 27.

It's possible, though, that Sewald is an even less likely name here, because while Vesia had little track record, Sewald's was just poor. In parts of four seasons with the Mets, he posted a 5.50 ERA, then last winter they chose to not even offer him a contract. Good for the Mariners, who signed him to a non-roster invite and watched him blossom into a relief ace. Thanks to suggested changes from the Seattle development staff, Sewald has a 39% strikeout rate, though he's allowed nine second-half homers after giving up zero in the first half.

Or maybe it's Whitlock? Drafted as a Rule 5 pick when the Yankees couldn't or wouldn't find room for him on the 40-man roster, Whitlock not only made the roster, he's thrived, striking out five in his Major League debut and not looking back from there. Over 72 1/3 innings, Whitlock has allowed only 16 earned runs while striking out 79. While he's currently out with an injured pectoral, he's expected to be back before the end of the season.

If you know Rogers, it's mostly because of his funky delivery, and that he's not Trevor or Taylor. But the focus on the way he pitches overlooks how he pitches, which is that in 78 innings this year, he's posted a 2.31 ERA. More importantly than that, no regular reliever has avoided the most dangerous of hard contact -- the barrel -- as well as he has. He won't pile up the strikeouts (a below-average 17% K rate), and that's not usually what managers look for in October, but just about no one prevents loud contact as well as he does.

In Canada, Romano (along with Adam Cimber, Trevor Richards and Tim Mayza) has been a consistent piece in a sometimes-shaky bullpen. Maybe, perhaps, we should have seen this coming, because his 2021 underlying stats look a whole lot like they did last year, when he had a 1.23 ERA (in only 14 2/3 innings). He's added three ticks on his fastball since this 2019 debut -- he now averages 97.6 mph -- and he's allowed just one hit in his last nine outings.

Tier 5: The solid veteran arms who will make some noise

Aaron Bummer, LHP, CWS
Kendall Graveman, RHP, HOU
Chad Green, RHP, NYY
Andrew Kittredge, RHP, TB

If you didn’t already know Bummer, you should; if you look at the ground-ball rate leaders since 2018, he’s second behind only Britton. He still gets the grounders, but he’s a lot more than that now; in the first three years of his career, he struck out 23% of the batters he faced. From 2020-21, that’s up to 32%. Bummer’s sinker may get the ground balls, but his slider is his true deadly putaway pitch.

Remember when Seattle traded Graveman to Houston and the players were so angry it ended their season? Yes, about that … anyway, Graveman got off to a dominant start with the Astros before running into some command issues that haven’t totally abated yet. (While his 1.04 ERA in September looks great, a 9/9 K/BB in 8 2/3 innings says otherwise.) In an attempt to better attack lefties, he’s using his slider four times as often against them this month as he did last month.

Green -- whose team hasn't clinched yet -- should get more respect than this, we think. He’s been a valuable Yankee for six years (3.19 ERA), he’s got more than five times as many strikeouts as walks the last two seasons and there was a stretch in early July, as the Yankees' season was at a low point, when he all but singlehandedly saved the bullpen. He’s allowed runs in five of his last dozen games, though, allowing homers in three consecutive.

Let’s be sure to give Kittredge his due, though, because after being a useful if not prominent arm for his first four years with Tampa Bay, he’s having something of a moment right now, and we’re not just talking about his last-second All-Star selection. He's not "1.66 ERA" good, but relying almost entirely on sinkers and sliders, he's posted a 75/15 K/BB for the Rays, particularly coming in handy against righty batters, with a dominant 56/5 mark in such spots.

Tier 6: The starting pitchers working out of the ‘pen

This quartet has a long history of being in the rotation; between them, they’ve made 370 career starts, including 36 this season. This isn’t the same thing as “team’s fourth- or fifth-best starter may work out of the bullpen in October,” as may happen with Adrian Houser or Shane McClanahan or Dylan Cease. These are the guys already working out of the bullpen, to great effect, though a couple of them are not yet guaranteed to pitch in October.

Luis Severino, RHP, NYY
Garrett Richards, RHP, BOS
Collin McHugh, RHP, TB
Reynaldo López, RHP, CWS

Yes, another Yankee, we’re sorry, but don’t forget that Severino finished in the top 10 in Cy Young voting in both 2017 and ‘18 before shoulder and elbow injuries ruined his next two seasons. Finally healthy but far from built up to a starter’s workload, Severino is a newly minted reliever for Boone, and so far, so good: Seven scoreless innings.

Further north, behold the transformation of Richards, who struggled so badly with grip issues after the substance crackdown that he lost his rotation job in the first week of August. Improbably, he’s been spectacular, continuing usage of his new changeup and posting a 1.93 ERA. (“Maybe this is the future for me,” he told The Athletic. If not the future, it’s at least the present.)

It’s a little different for McHugh, a long-time starter who didn’t pitch in 2020 and signed with the Rays without a defined role. He quickly became a valued part of the bullpen, though he’s still started eight times this year, or at least “was the first pitcher” eight times, given how fluid Tampa Bay pitching spots can be. Wherever he’s pitching, he’s been strong, with a 1.60 ERA and a 73/12 K/BB.

From 2016-20, no one started more games for Chicago than López did, but he spent most of the first half injured or in the Minors, and he’s spent September pitching in as a starter to get the rotation through the end of the season. But in between, López worked in relief, and the results were spectacular; in 20 innings as a reliever, he allowed two earned runs, striking out 22.