2024 in one sentence

After serving as the sacrificial lamb in an early-season loss to the Mariners, Justin Slaten bounced back to all but secure the spot as Boston’s shoo-in closer following Kenley Jansen’s departure from Boston.

The Positives

Slaten’s rookie season was one of the best seasons that we’ve seen from any Red Sox reliever in recent memory. While the closer duties were largely left to the veteran Jansen, his late-season stint on the injured list gave Slaten the chance to slot right into that closer spot at the end of the year, which should transition well into that role next season, depending on Liam Hendricks’ status and continued recovery over the offseason.

Slaten’s save in Boston’s last game of the season was only his second of the year, but he picked up 13 holds and six wins in 55.1 innings out of the bullpen. While all of these results are great, I think the most important numbers to showcase Slaten’s effectiveness are not in the game results he generated, but the individual results that his stuff generated in plate appearances.

He finished the year with a 58:9 K:BB and in the 97th percentile for BB% and Chase%. While his Whiff% finished at “only” the 76th percentile, Slaten was able to effectively generate soft contact on these chases that didn’t end up being whiffs. His Savant profile was a thing of beauty, as he finished above average in every single category and finished above the 70th percentile in 12 of 13 statistics. I’m excited to see what Slaten can do after a full offseason of major-league work, and he’s a great bullpen piece to have to return to a questionable group in 2025.

The Negatives

The one thing that Slaten will have to improve on as his stuff is seen more in the majors is his breaking ball… and the four blown saves. Slaten’s cutter and four-seamer performed well together (he ranked in the 88th percentile in fastball run value), but his breaking ball combination (a sweeper and seldom deployed curveball) did not hit the mark of a major league quality breaking ball duo.

He deployed the sweeper 24.7% of the time, his third-most used pitch, and allowed a .322 wOBA over the course of the year. Opposing hitters hit .279 on the sweeper, which was his least consistently grouped pitch.

Slaten’s cutters and four seamers were both grouped pretty tightly across their high usage, but the sweeper was a bit all over the place in terms of induced break over the course of the year.

Slaten also had some rocky moments in his test drives in the closer role, but I would be less concerned about that as he gains more experience and is deployed in that role or an eighth-inning role more consistently. Still, four blown saves in a limited amount of save chances.

Best Moment

My favorite Slaten moment of the year had to be when he secured his first career save on May 31st after blowing a save just a couple of months before in his MLB debut.

While that blown save against the Mariners really wasn’t his fault (he was largely shoved into a game where Boston ran out of relievers to face a good offense in a late-night West Coast game for his first appearance as a Red Sox), it was nice to see him bounce back at a time when the Boston bullpen truly needed his help.

The Big Question

Can he fix his breaking balls to pair with the elevated heaters?

2025 and Beyond

No matter where Slaten is slotted in this upcoming year’s bullpen (dependent on Hendricks and any offseason signings that may occur), his biggest focus this offseason needs to be on workshopping these breaking balls to be effective and useful against big-league hitting. One option to do this would be either deploying his curveball more (which was rarely used, but recorded his lowest batting average against on a single pitch when it was) or workshopping that sweeper to be more controlled. Both of these should up his already high chase rates, and if he’s able to effectively pair them since they’re different enough, it could make his already threatening heater arsenal that much scarier with a competent breaking ball duo.

Boston doesn’t have the best record when it comes to redeveloping breaking pitches over the offseason, but recent developments from the likes of Tanner Houck and Nick Pivetta give me hope that a significant change could come for Slaten with this offseason’s work.