I’m almost out of pitchers to write about, I swear. We’re moving on to the middle relievers. The guys who sometimes need to take it on the chin, who rarely get the credit they deserve, but oftentimes make or break close games.
The Red Sox’ starting rotation depth and quality should help keep this group fresher than they’ve been in the recent past. Right now, I think this group might be on the light side, but relievers spring up out of nowhere and are available at the drop of a hat, so a trade or the addition of a DFA’d arm from another team could add quality to the bullpen. Here’s a look at way too many pitchers who might get a chance for the Red Sox this year.
Justin Slaten
Justin Slaten’s 2025 was disappointing, in large part because he was unavailable for most of the middle part of the season. He posted a 4.24 ERA with an 18.2% strikeout rate and 7.3% walk rate. Those numbers were a large step down from his rookie season, but the underlying stuff was just as good.
His fastball in particular was excellent. It has solid vertical movement and great velocity with big extension. In 2025, the swinging strike rate was among the best in the league at 15.6%, while the ideal contact rate was a great 31.3%. His cutter was a reliable strike-getter and also limited hard contact. Slaten struggled to put lefties away in 2025 due to leaving his curveball in the zone with two strikes too frequently, but better command should see the pitch return strikeouts. His sweeper was useful in putting away righties and should continue to be a weapon for him.
The issue for Slaten was getting out of trouble. His left-on-base rate of 55% was remarkably low and should rise to a normal rate over a full season. His 2024 rate was 71%, and there’s little reason to believe it shouldn’t normalize given his stuff. Aroldis Chapman and Garrett Whitlock are the top dogs in the Red Sox bullpen, but Slaten will be asked to handle high-leverage situations regularly.
Greg Weissert
Dear Alex Cora, I know navigating the middle innings can be difficult, particularly in a tight game. Hopefully, your rotation gives you six innings often enough that nobody in the pen needs to be overworked. I’m writing to you today to plead that you don’t give the ball to Greg Weissert every night. I know it’s tempting, but don’t do it.
Each of the last two seasons, Weissert has been near the top of the league in appearances through June, and then seen his performance tail off in the summer before reemerging at the end of the season. Weissert commands his fastball at the top of the zone, where it plays well due to its flat approach angle. Against righties, he uses a sinker and slider/sweeper to great effect. Lefties are a problem, with nothing else besides a fastball to get strikes with, though. He’s best used matching up with righties in a one-inning spot, and shouldn’t be asked to do much more.
Zack Kelly
I don’t think I’ll ever be able to quit Zack Kelly.
See?
The stuff is just too good not to work. His four-seam averages 96 mph with seven feet of extension. The pitch has a flat approach angle, though I wonder if it’s a little “dead zoney”, because righties have been able to handle it. It was primarily a two-strike pitch and returned whiffs due to solid command, but the contact against it was hard. Early in counts, he used a sinker and a cutter that each earned strikes and limited damage. His sweeper also punched out hitters, but like the four-seam fastball was hit hard. Lefties got four-seams and cutters for strikes, with changeups deeper in counts. The slow ball strike rate was an abysmal 49% to lefties, and needs to see improvements to become a viable weapon.
Kelly’s approach against righties is sound, while he needs an “out-pitch” against lefties. Despite this, his OPS allowed was .831 against right-handed hitters and just .604 against lefties. I’ll chalk this up to bad luck. He struck out 29.3% of righties compared to 15.7% of lefties. His batting average on balls in play was over 0.400 against righties and is sure to come down to a more normal rate. His left on base percentage is also sure to come up, unless there are some underlying issues with the stretch that I’m not seeing. If you’re out on Kelly, I don’t blame you. After looking into the numbers, I’ll go the other way and predict that Kelly will be a key member of the Red Sox bullpen by the end of the season.
Ryan Watson
Ryan Watson was a Rule 5 draft pick, which gives him a solid chance to break camp with the team. He won’t be used in high-leverage spots, which means his job is to throw strikes. If he can throw strikes and handle low-leverage work, he’ll potentially be used in more meaningful spots later in the season. In the minors, he used a fastball, sinker, slider, and curveball. His four-seam hasn’t missed bats in the minors, but his low-release and above-average extension should allow the pitch to play at the top of the zone. His slider and curveball have each shown the ability to create whiffs as well. In his first spring training outing, he featured a low-90s cutter as well that has potential. The stuff doesn’t jump off the page, but he has weapons to attack both sides of the plate and provide an inning or two of relief at a time.
Kyle Keller
Of the non-roster invitees, Keller has the best chance to make the roster given the financial commitment. He’s in line to make $1.8 million if he breaks camp with the team. I don’t know a ton about Keller; he’s spent the last several seasons in Japan, most recently with the Yomiuri Giants. He has a fastball, cutter, curveball mix, with the four-seam making up the bulk of his arsenal. It comes in at 151.7 kilometers per hour, which sounds pretty fast, but is actually about 94 mph. His secondary pitches were used in a small sample, as far as I can tell, with middling results. He also walked three men in his second spring training outing, which you can’t afford to do as one of the lower-leverage arms in the bullpen. The Red Sox are willing to pay him if he makes the roster, so there’s clearly something they like, though.
Tayron Guerrero
Tayron Guerrero wasn’t a name I knew until Christopher Smith wrote about his journey to Boston, but now I’m all in. He’s a 6-foot-8 righty with massive velocity and command issues. He started playing baseball in 2007 after watching the Red Sox win the World Series, and was in the major leagues less than 10 years later. He hasn’t been able to stick with a big league team, bouncing around from organization to organization and spending time overseas, but his velocity gives him a chance to be special if he can find the zone. He’s using a four-seam and a two-seam that each get up to 100 mph, a splitter, and is working on a gyro-slider as a way to be in the strike zone more often. He’s on a minor league deal and is unlikely to make the opening day roster, but if he proves the ability to throw strikes, he could climb the bullpen trust tree and end up in high-leverage spots. Of the names at the bottom of this list, Guerrero has the highest ceiling.
Seth Martinez
Did you like John Schreiber? If so, you’re gonna love Seth Martinez. Martinez spent last season with the Marlins, primarily in Triple-A. Before that, he made 111 appearances for the Astros from 2021 to 2024, registering a 3.93 ERA over that time. He’s a side-armer with big extension who tops out about 90 mph, using a sinker, cutter, four-seam, sweeper, and changeup. His fastball is super flat, and in theory, should play at the top of the zone, while his sinker can work to jam righties. His sweeper and changeup have each shown bat-missing ability in the past, but his lack of velocity caps his ceiling and makes command paramount. He’s another non-roster invitee with an outside shot to make the team.
Noah Song
He exists! He’s thrown a few single innings in relief so far this spring, and his slider looks sharp. His fastball has high vertical movement, and his changeup shape is super interesting. *Insert Rick Reilly-style joke comparing the length of Noah Song’s story to a Grateful Dead song.* It’s probably a long shot at this point, but I could be convinced.
Vinny Nittoli
Aye! I’m relievin’ here! Nittoli is a non-roster invitee with some major league experience. He uses primarily a cutter that doesn’t actually cut, from a low arm angle. It’s not something I’ve seen before, which could be a classic case of survivorship bias, but it could also provide a look that hitters haven’t seen before. He uses a sinker, changeup, and sweeper as well. He’s kind of like Cooper Criswell, but more Italian-sounding.
Devin Sweet
Having different looks out of the bullpen is important. The Red Sox have an affinity for low-slot pitchers, while Devin Sweet is the opposite. He’s got a very high arm angle that creates a ton of vertical movement on his fastball. He also has a cool changeup shape that has a huge velocity and movement separation from that fastball. Early Spring Training hasn’t gone well for him, and he’s had problems throwing strikes, but there’s some real swing and miss stuff if he can harness it.
