A year ago, Brandon Morrow was an oft-injured non-roster invitee for the Dodgers. Chad Green was Minor League rotation depth for the Yankees. Corey Knebel was coming off a 4.68 ERA season and planning to set up for Neftali Feliz. Anthony Swarzak was a non-roster invitee coming off a 5.52 ERA. Mike Minor had thrown zero big league pitches since 2014.
They weren't notable names, nor were they expected to play big roles. Plus, if you're really being honest with yourself, you didn't even know who Richard Parker, Kirby Yates or Tommy Kahnle were. 
If we'd put that list of names together in February 2017 and told you that they all would become extremely valuable relievers in the year to come, you'd probably have laughed, but that's exactly what happened. Minor, Morrow and Swarzak all signed multiyear free-agent contracts. The others had great years, several for playoff teams, and are being counted on for more in 2018.
So who will be next to break out? Here are some candidates to look out for in the coming season.
The former starters
Adam Morgan, Phillies
Tyler Lyons, Cardinals
Tom Koehler, Dodgers
As with Andrew Miller, Wade Davis, Brad Hand, Swarzak, Morrow and so many others before them, a good way to find a pitcher who can succeed in relief is to look for those who couldn't keep it together in the rotation, either due to health, repertoire or stamina.
Morgan was a formerly well-regarded prospect who missed all of 2014 after rotator cuff surgery, then he arrived in the big leagues with below-average velocity, throwing fastballs at 89.6 mph in '15 and 91.4 in '16. That didn't cut it; he had a 5.37 ERA working almost exclusively as a starter. In '17, Morgan became a reliever, and he started throwing 95, though the larger change was that he threw the fastball less and his good slider and changeup more. He ended up with a 63/18 K/BB in 54 2/3 innings, and by one major Statcast™ metric, he was a Top 20 reliever in the second half.

Lyons has been kicking around the bigs with the Cardinals since 2013, usually as a sixth starter and swingman, and then he missed the end of '16 and start of '17  after knee surgery. When he returned, he was quietly very good, throwing his slider 51 percent of the time -- more than any regular lefty aside from Miller -- and striking out 68 in 54 innings with a 2.83 ERA.
Koehler was not good in 2017. There's no getting around a 6.69 ERA, which he had for Toronto and Miami, or a career 4.39 mark. But when Koehler arrived in Toronto, the Blue Jays made him a reliever, and he was much better. In 12 relief innings, he struck out 11 with a 3.00 ERA, flashing improved velocity and throwing more than 80 percent fastballs and curves after doing so just 60 percent as a starter. The Dodgers took a chance on Koehler with a one-year deal in hopes of him becoming the next Morrow.
The import
Chris Martin, Rangers
You probably don't remember Martin putting up a 6.19 ERA in 40 games for the 2014 Rockies and '15 Yankees, but that's OK; we don't either. Martin spent the past two years in Japan, and the numbers in 92 games were stunning: a 1.12 ERA in 88 1/3 innings. Of the 190 pitchers who threw 50 innings in Japan over the past two years, Martin had the second-largest gap between strikeout rate and walk rate. That was good enough for Texas, which signed him to a two-year deal in December.
The traded relievers you didn't notice were already good
Emilio Pagan, A's
Dominic Leone, Cardinals
Both pitchers were involved in trades this offseason, which tells you that their new teams saw something they liked. The A's picked up Pagan from Seattle for Ryon Healy, just one of a trio of new Oakland relievers, along with Ryan Buchter and Yusmeiro Petit, which is part of why we find the A's so interesting this year. Leone came to St. Louis from Toronto in the Randal Grichuk deal.
Pagan struck out 56 in 50 1/3 innings and did so with so many weak flies and popups that he was a Top 12 reliever by the primary Statcast™ quality-of-contact metric. Leone whiffed 81 in 70 1/3 with a 2.56 ERA; Buchter had a 2.89 ERA while striking out a batter per inning and finishing in the Top 30 by the same metric.
None of them are big names. None are All-Stars. That could change this year.
Video: BAL@TOR: Leone fans Trumbo for his first MLB save
The 28-year-old rookie
Kevin Shackelford, Reds
A 4.30 ERA from a pitcher who will be 29 in April and spent parts of eight years in the Minors before making it to the big leagues doesn't exactly scream "stardom," but that's exactly who we're looking for. Shackelford piled up some big numbers in his 30 2/3 innings, striking out 38, after putting up a 1.53 ERA and nearly 12 whiffs per nine innings in Triple-A. A few bad early games inflated the ERA, but when looking at his quality of contact, he was more like Chris Devenski or Cody Allen -- in a much smaller sample, of course.
The former high pick who just needs to stay healthy
A.J. Minter, Braves
Minter was a second-round pick in 2015, but he's thrown only 74 professional innings due to Tommy John surgery, a separate elbow strain and a groin strain. During the rare times he's been on the mound, however, he's been phenomenal, using his 96-mph fastball to whiff 77 in 59 Minor League innings. Minter then put up ridiculous numbers in a brief Major League stint last year: In 15 innings, he whiffed 26 and walked just one. Last year, 607 pitchers threw 10 innings or more. Only one had a higher strikeout percentage: Craig Kimbrel. That'll work.

The mid-season 2017 callups you definitely don't know
Drew Steckenrider, Marlins
Trevor Hildenberger, Twins
Did you know Steckenrider struck out 54 in 34 2/3 innings, a Top 15 rate among relievers with 30 innings? Like some others on this list, he entered the pros as a starter, then missed a full year due to Tommy John surgery. In 2015, Steckenrider's final year as a starter in the Minors, he struck out seven per nine innings. Then in '16, his first year as a reliever, he whiffed 12 per nine innings. That carried over in the big leagues, and Steckenrider fits the profile of a dominant reliever in so many ways, from velocity (95.6 mph) to simplifying to a fastball/slider repertoire.
The sidearming Hildenberger has always been a reliever, but his calling card has been outstanding control. In 171 2/3 Minor League innings, he struck out 200 and walked just 26. Hildenberger doesn't throw hard -- just 88.8 mph, actually -- but because he gets grounders, misses bats and doesn't walk anyone, the outcomes have been good. He's also on that same Top 30 list, per Statcast™ quality-of-contact metrics.