Bullpens have gained prominence but, alas, not predictability. Even the savviest of executives will tell you that estimating bullpen performance is an inexact science within the larger inexact science of predicting performance in general.
All we can do as we continue our series of Top 10s leading into Opening Day (we began Tuesday with the Top 10 lineups) is operate off reasonable projections, knowing full well that the season will surprise us -- as it did in 2017, when guys like Corey Knebel, Chad Green, Brandon Morrow and Archie Bradley blew past their personal projections to become some of the most valuable relief arms in the game.
So here's a look at my picks for the 10 best bullpens going into the regular season. (And like a late lead in the hands of a bumbling 'pen, you can probably expect this list to get blown up before long.)
Closer: Aroldis Chapman
Top setup options: David Robertson, Dellin Betances, Tommy Kahnle, Green
If this relief crew holds up, it will feature the most devastating strikeout stuff in the game. Even in what can be construed as a "down year" for Chapman and Betances in 2017, they K'd 12.3 and 15.1 batters per nine, respectively. Green struck out 13.4 per nine in his emergence as a multi-inning option. Robertson returned to the organization to post a 1.03 ERA in 30 appearances, and in 2018 he might not even be the best addition from that trade with the White Sox (Kahnle has the K rate and the potential to be every bit as good).
Video: Outlook: Allen looks to be consistent reliever again
Closer: Cody Allen
Top setup options: Andrew Miller, Nick Goody, Dan Otero, Zach McAllister
What the Indians lost in free agency is obvious: Bryan Shaw's 72-inning average over the last five years made him extremely valuable and Joe Smith was an impactful in-season acquisition in 2017. But what the Tribe retains is equally obvious. Allen and Miller (both pending free agents themselves) are as good a closer-setup pair as exists in the sport, and Miller's elite application in any inning, anywhere, anytime amplifies his impact (Miller is projected by FanGraphs to be worth 2.1 WAR, more than the entire bullpens of the Marlins, Royals and Tigers).
Video: Outlook: Giles should be one of AL's top relievers
Closer: Ken Giles
Top setup options: Chris Devenski, Will Harris, Hector Rondon, Smith
The Astros used smoke, mirrors, Lance McCullers Jr. and Charlie Morton to get their biggest outs of the postseason. Hard to know how much to read into the October 'pen vs. the regular-season 'pen, but it's generally best to bet on the bigger samples, and a strong, deep rotation could take some pressure off the relief corps. Giles had a 2.30 ERA and a 1.04 WHIP in 62 2/3 regular-season innings last year. A big key will be Chris Devenski providing similar value to what he gave Houston in 2017, when he ate up 80 2/3 innings with a 0.94 WHIP.
Video: Outlook: Doolittle can be elite closer if healthy
Closer: Sean Doolittle
Top setup options: Ryan Madson, Brandon Kintzler, Shawn Kelley, Koda Glover
Not long after Dusty Baker threw up his hands and said, "We need some help, big time," Nats president of baseball operations Mike Rizzo acquired multiple years of control of Doolittle and Madson and traded for (and subsequently re-signed) Kintzler. Bada bing, bada boom. What once was a clear weakness is now a strength, even if Baker is no longer around to enjoy it. Any one of those three fairly recent additions is capable of closing out a game, and so Washington no longer has to thrust a kid like Glover -- who has been dealing with right shoulder issues -- into the ninth ahead of schedule.
Video: Shaw, McGee, Iannetta, Davis and Black on 'pen depth
Closer: Wade Davis
Top setup options: Mike Dunn, Bryan Shaw, Adam Ottavino, Jake McGee, Chris Rusin
The Rox basically bought their way onto this list (though I can assure you not a single dollar went to the author). Their expenditures on Davis, Shaw and McGee, combined with what they's already committed to Dunn and Ottavino, mean they'll be investing one-third of their payroll on five relievers. I'd certainly like this arrangement a lot better if Colorado was getting 2014-15 Davis. And because of his aforementioned workload the last five years, Shaw might be a ticking time bomb. But if healthy, the Rockies will be effective, and multi-inning man Rusin was an unsung hero last year.
Video: Jansen on increased workload, preparing for season
Closer: Kenley Jansen
Top setup options: Scott Alexander, Josh Fields, Tony Cingrani, Pedro Baez
Do I know for certain how the bullpen will look and perform beyond Jansen? No. Even manager Dave Roberts recently called the competition in the back end of the bullpen "fuzzy" after would-be Morrow replacement Tom Koehler suffered a right shoulder injury. But Jansen struck out 109 batters and allowed just 44 hits and seven walks in 68 1/3 innings last season. His presence alone is enough to earn the Dodgers a spot on this list. The interesting wrinkle with the Dodgers will continue to be the management of starters' workload, as those not named Clayton Kershaw are often given quicker hooks here than elsewhere. That adds to the importance of the 'pen.
Video: Jansen, Kimbrel among top fantasy relief pitchers
7. Red Sox
Closer: Craig Kimbrel
Top setup options: Joe Kelly, Carson Smith, Matt Barnes, Tyler Thornburg
As with the Dodgers, a truly elite closer carries weight here, and Kimbrel (1.43 ERA, 126 strikeouts and just 33 hits and 14 walks allowed in 69 innings in 2017) is obviously that. I do feel there are probably more questions in front of Kimbrel than there are in front of Jansen. Kelly has long had sick stuff with sometimes-sickly results, but the way he settled into the relief role as 2017 evolved (2.79 ERA) bodes well. Maybe this will be the year the Red Sox get what they traded for in Smith (2.31 ERA in 70 innings in '15) and Thornburg (2.15 ERA in 67 innings in '16), but of course, there's no telling.
Video: Morrow on taking over closer role, 2017 postseason
Top setup options: Carl Edwards Jr., Steve Cishek, Pedro Strop, Justin Wilson
Forget adjusting to the ninth. The bigger question with new closer Morrow is whether the workload he took on last year (he went from 82 1/3 innings combined from 2014-16 to 57 1/3 between the regular and postseason in '17), especially in pitching all seven games of the World Series, affects him at all in the new year. And it remains to be seen if holdovers like Edwards and Strop can limit the wildness that pervaded this unit last year. But the additions of Morrow and Cishek do give it added depth and dimension, and prospect Dillon Maples could emerge as an important piece.
Video: Outlook: Knebel looking to build on breakout campaign
Top setup options: Josh Hader, Matt Albers, Jeremy Jeffress, Jacob Barnes
Last year, in what looked to be a time of major transition after the trade of Thornburg and despite a rough start to the season, the Brewers' bullpen was surprisingly the backbone of the club's surge up the standings, with Knebel's emergence in the ninth (1.78 ERA, 39 saves) a big key. Knebel has not had a sharp spring, but it's always hard to know how much to reach into those numbers. It's easy, on the other hand, to be excited about the potential that accompanies a full season of Hader (2.08 ERA in 47 2/3 innings last year). The Brewers lost Anthony Swarzak in free agency, but they brought in Albers on the cheap after his sub-2.00 season in Washington.
Closer: Jeurys Familia
Top setup options: Anthony Swarzak, AJ Ramos, Jerry Blevins, Paul Sewald
Will new manager Mickey Callaway's progressive pitching plans uncover greatness or reveal weakness in the 'pen? The big key will be Familia returning to normal after last year's bout with a blood clot in his right shoulder led to him missing 3 1/2 months. The signing of Swarzak, who had a 2.33 ERA and 1.03 WHIP across 77 1/3 innings with the White Sox and Brewers last year, is big, as was the 2017 acquisition of Ramos. If the Mets' starters generally stay healthy (yes, that's a huge "if"), they can apply their depth here to a high-leverage multi-inning relief role -- a la what the D-backs did with Bradley last year.
Anthony Castrovince has been a reporter for MLB.com since 2004. Read his columns, listen to his podcasts and follow him on Twitter at @Castrovince.