MLB Relievers Who Could be Successful Starters in 2025
MLB Relievers Who Could be Successful Starters in 2025
Chris Clegg discusses what relievers could make a transition to the rotation in 2025 and what traits we should look for in those arms.
Every year, we see relievers take a step forward in a variety of roles. Some make huge jumps to become elite closers like we saw from Kyle Finnegan and Robert Suarez this year. Others, such as Garrett Crochet, Reynaldo Lopez, and Seth Lugo, have made jumps in the last two years to go from relievers to successful starting pitchers.
Identifying relievers who could become viable starting pitchers can be huge for you in a fantasy setting, as the cost is likely depleted. It is also usually common for a lot of bullpen arms to be starting pitchers coming up through the minors before being “demoted” to a bullpen role.
While the move from the bullpen to rotation is not something we used to see often, we have seen much more success in recent years, and teams are doing a great job at identifying arms that could make that jump.
Who Has Recently Succeeded in the Transition?
In 2023, Seth Lugo made the jump to the rotation. This year with the Royals, he was a top-six pitcher in baseball based on WAR and threw 204.2 innings, the current lead in baseball.
After being a starter at the University of Tennessee, Crochet was immediately thrown into the bullpen in Chicago during his draft year. After missing time with Tommy John, Crochet transitioned to the rotation in 2024 and despite only pitching 142 innings, he dominated. Crochet posted a 3.68 ERA with xERA, FIP, xFIP all below three. He struck out 35 percent of batters faced and had the eighth-highest WAR of any pitcher in baseball.
Michael King moved to a starter role in late 2023 with the Yankees and thrived. He was traded to San Diego and has dominated across 168.2 innings with a 3.04 ERA and a 28 percent strikeout rate.
Nick Martinez transitioned to be a starting pitcher down the stretch this year for the Reds and has thrived. He was featured in my pitchers to buy heading into the offseason. Martinez has shown the ability to eat innings and has posted a 2.72 ERA across 56 innings over that span. In his last three starts, Martinez has tossed 19 innings of one-run ball with 20 strikeouts and just one walk.
The Braves told everyone when they signed Lopez to a 3-year/$30 million deal last offseason they intended to make him a starter. To put it nicely, Lopez had not been good as a starting pitcher and has finally found a groove as a reliever in 2022 and 2023.
In 128.2 innings in 2024, Lopez posted a 2.03 ERA and struck out 26 percent of hitters while walking just eight percent. Those names I just listed were all top 30 pitchers based on fWAR this year who all transitioned from the bullpen to rotations.
In a Stuff+ Primer by Owen McGrattan on Fangraphs, we see that a reliever going to a starter should see around a 5.5-point drop in Stuff+. Ten points in Stuff+ is a standard deviation, so the typical reliever does not see a full standard deviation drop. On an individual pitch level, you can see the averages and standard deviations below.
Looking at the Stuff+, Pitching+, and Location+ of these arms as relievers and starters, we see a major fall in stuff from those who had big-time stuff. With Lugo and Martinez, who did not have big stuff out of the bullpen, there is not a substantial drop. Location+ and Pitching+
So, what can we learn from these arms when looking for pitchers who could make a jump from relievers to starters?
For the most part, these arms have really good stuff across the board or at least several above-average offerings. With Lugo, even though the stuff does not jump off the page overall, he has a deep arsenal and positive run values on all of his pitches due to unique traits on them.
Another important trait is strike-throwing. Can they throw strikes over a one-inning stint? A.J. Puk was a failed experiment as someone who attempted to move to a starter role. His command was not there for more than an inning at a time. Four starts and 9.22 ERA later, and he went back to the bullpen, where he has thrived. Good command and strike-throwing is essential for a reliever jumping to the rotation.
It is also highly important for a pitcher to have three or more pitches. Sure, two pitches work for some, but Spencer Strider’s of the world who have two 70-grade pitches are few and far between. He even worked on adding a curve this season.
Often, two-pitch arsenals are in the bullpen because they lack the ability to get a certain-handed batter out. Being able to get out both lefties and righties is crucial for success as a starter.
Puk, for example, had a .261 batting average allowed and a .765 OPS allowed against righties in 2023. In his short stint as a starter, a .958 OPS against righties. In 2023, Puk had a .578 OPS allowed against lefties, and while it was a very small sample as a starter, the number jumped to 1.265.
Jump back to Puk as a reliever in 2024, and he has a .327 OPS against left-handed hitters and a .414 against righties.
So, who might make the transition to being a starter in 2025, and can they be successful?
Relief Pitchers who Could be Solid Starters in 2025
Lucas Erceg, RHP, KC
Erceg has quite a good year in Oakland before the trade, mostly pitching in the setup role behind Miller. In 36.2 innings with the Athletics, Erceg posted a 3.68 ERA with 41 strikeouts in 36.2 innings. After the trade to Kansas City, Erceg dominated, posting a 3.27 ERA in 22 innings with a stellar 1.26 FIP. He struck out 27 and walked just three batters. He recorded eight saves and six holds in the matter of a month and a half period.
Upon the trade, Erceg upped his slider usage significantly, and saw his four-seam usage drop around six percentage points. The changeup is lowest graded Stuff+ pitch and we have seen that usage take a big of a dip.
Having a 107 Stuff+ and a 104 Pitching+, the slider grades out as his best offering, while the four-seam and sinker both sit around 110, a standard deviation above average.
The most notable thing is his strike and walk rate improvements from Oakland to Kansas City.
There has been some speculation that Erceg could make the jump to be a starter next year in Kansas City. They have had success with Seth Lugo, so keep an eye out here on this situation.
Mason Miller, RHP, OAK
Miller has been an elite reliever this year, there is no argument against it. In 61.2 innings, Miller has 27 saves with a 2.34 ERA and 97 strikeouts. Striking out over 41 percent of batters faced with the fourth best Stuff+ in baseball is nothing to scoff at. He has been a starter before, but can he handle full seasons workload?
The fastball averaged 101 mph this year, with one of the top whiff rates among any fastball. The Stuff+ on that pitch sits at 137, pairing it with a slider with an absurd 160 Stuff+ and a 47 percent whiff rate against.
Miller lacks a true-third pitch but did throw a changeup 16 times this season, all against lefties. He did throw three splitters, which is a pitch I would love to see him develop further.
It is pure speculation here that Miller could be moved back to a starter role, but here is a case for it:
The lineup ended up being much better than anticipated, and the team performed down the stretch. Since July 1, the Athletics had the 10th-best record in baseball at 37-33. Their roster severely lacks an upside starting pitcher. The current rotation of JP Sears, Joey Estes, Mitch Spence, Brady Bassom, and J.T. Ginn does not inspire much confidence. The team is not likely to sign a top-end free agent, nor are they likely to trade for an ace. The team’s best shot is Mason Miller. Do they stretch him out this offseason? We shall see.
Cade Smith, RHP, CLE
Smith was a part-time starter at the University of Hawaii, but you would need to go back to 2020 to find the last time he started a game. Making his MLB debut this season, Smith looked like one of the best pitchers in baseball out of Cleveland’s bullpen. He tossed 73.1 innings with a 1.96 ERA and a 0.91 WHIP.
Smith struck out 35.5 percent of hitters to this point with just a six percent walk rate. The 119 overall Stuff+ was high-end, and he did it while throwing strikes at an impressive 68.5 percent clip.
The 25-year-old does have three pitches in his four-seam, splitter, and sweeper, which all missed bats at a solid clip. The splitter grades out as one of the best in baseball with a 136 Stuff+, and he pairs with a well above-average grading fastball and a slider that has a 126 Stuff+.
Smith’s locations are very strong, and his splits between lefties and righties are very good. Against lefties, a .169/.230/.234 slash allowed with a 38.5 percent strikeouts rate. Very strong against righties, he struck out 32.7 percent of hitters with a .212/.265/.321 slash against.
There is no evidence Cleveland plans to stretch him out. While Cleveland has a ton of arms that are close to ready for the Majors, they have vets in Alex Cobb and Matt Boyd coming off the books. With Shane Bieber on the shelf for most of next season, Gavin Williams and Tanner Bibee are the only true locks for the rotation. Could Cade Smith get a chance?
Adrian Morejon, LHP, SD
Morejon was a starter his entire Minor League career before converting to a full-time reliever in 2022. 2024 turned out to be his best season, as he made 57 appearances, throwing 62 innings with a 2.90 ERA, a 26.7 percent strikeout rate, and a 7.6 percent walk rate.
He completely reworked his arsenal, curbing his four-seam for a sinker and using his slider at an 11 percent higher rate than he did in 2023. In the process, Morejon transitioned his changeup to a splitter and added a sweeper that he threw on occasion.
The four-seam got hit extremely hard in 2023, but the sinker has just a .289 xSLG against this season, generating a ton of ground balls. The splitter and slider have whiff rates of 51.5 and 39.9 percent, respectively. Having three pitches with Stuff+ grades north of 123 is extremely impressive.
As a left-handed arm, he gets both left and right-handed hitters out effectively and throws plenty of strikes. The Padres have proved they will pay for any starter they want or go out and get one via trade such as Dylan Cease. Could they stretch out Morejon? Potentially, but that is still to be determined.
Justin Slaten, RHP, BOS
Slaten was selected by the Mets in the Rule 5 Draft just several picks before Boston was due to pick. They liked him enough where they immediately traded 2023 draftee Ryan Ammons for him.
Despite only having eight innings above the Double-A level entering the season, Slaten shoved as part of the Red Sox bullpen, posting a 3.04 ERA across 51 innings. He struck out 26.5 percent of hitters he faced while walking just 4.2 percent.
The Stuff+ is not as high as the other arms on this list, checking in at 109 overall. Slaten has a true four-pitch mix with the cutter, four-seam, and sweeper being used at least 25 percent of the time each. The curve is used more often against lefties, but all four pitches were highly effective.
The four-seam checks in at 113, with the slider at 114 from a Stuff+ standpoint. The curve was only used eight percent of the time and was graded as the second-best in baseball, only behind Joe Musgrove.
Slaten throws strikes at a 67 percent rate and has the arsenal to get out both lefties and righties effectively. A wild stat is that Slaten only walked 0.9 percent of right-handed batters he faced this year. No, that is not a typo.
The Red Sox' pitching development has become quite good, and we have seen them effectively transition other arms to the rotation. I would not be shocked if Slaten is stretched out this offseason and joins the rotation in 2025.