Article: Justin Slaten's Success Comes From A Uniquely Altered Pitch Mix

Alex Mayes · May 1, 2025

The Red Sox seem to have a knack for identifying relievers who can contribute on the big league roster in the Rule 5 draft. First, Garrett Whitlock dazzled us in 2021. Now, Justin Slaten looks like the closer of the future in Boston. So far this season, Slaten has been working in tandem with Aroldis Chapman in the closer role, but if he continues to dominate like he has been, there’s no reason to believe that he won’t fully take over the ninth inning gig at some point during the 2025 season. Let’s take a look at the underlying factors that have made him so dominant this year.

First, Slaten has favored his four-seamer over his cutter so far in 2025. In 2024, Slaten featured a cutter 38% of the time and a four-seamer 29% of the time. This year, though, he’s essentially flipped those pitches. He’s throwing his four-seamer 39% of the time and his cutter 28% of the time. He’s throwing both offerings with a bit more speed, too, as the four-seamer is up from 96.4 mph to 97 mph and the cutter is up from 91.5 mph to 92.8 mph.

The benefit here is that these pitches tunnel out of the exact same spot and it’s incredibly difficult to pick up the movement on the ball until it’s too late. What’s more interesting though, is that in 2024, both versions of the fastball were pitches that he would go to when he was ahead in the count. He was spotting the pitches in different locations and having a ton of success with his fastball offerings. This season though, his off-speed offerings have become his bread and butter.

Last year, Slaten threw his sweeper 25% of the time, while his curveball barely saw any usage, sitting at just nine percent. As aforementioned, though, he's made a pretty big change this season. His curveball has jumped to a 21% usage, and his sweeper has fallen to nine percent.

Part of this can be explained by the shape of the sweeper. So far this season, his sweeper features more rise than it did last year. Typically, sweepers that rise in the zone get knocked around fairly easily by major league hitters. Maybe this was a purposeful, mechanical change, or perhaps it was an error in his delivery. Regardless, the righty has made a conscious choice to focus on his curveball, and that has proved to be the key to his early season success.

Slaten's hammer is absolutely nasty. It’s averaging 6.3” more vertical drop than other right-handed pitchers in 2025. When hitters swing at it, they flail at something that is virtually unhittable. Just take a look at this tweet from Rob Friedman to see how much Slaten's curve moves.

That's just downright unfair. The eye test may not be as valuable as it once was, but as a baseball fan, it's still a fun thing to watch such a ridiculous pitch. Slaten has already gotten five whiffs on the curveball, almost half of his total from last season with the offering (11). He hasn’t allowed a hit on the curveball this season, and if it keeps moving like it has been, it could be a long time before someone makes solid contact with it.

Justin Slaten has arguably been the best reliever in the bullpen since last season and he looks to continue his dominance in 2025. As he proves to be an elite reliever with a fluid pitch mix, look for him to secure the closer’s job as the season continues.

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