Yankees are seeing this under-the-radar free agent signing blossom in bullpen

MLB: New York Yankees at Arizona Diamondbacks
Mark J. Rebilas-USA TODAY Sports

Luke Weaver was signed by the Yankees to a one-year $2 million deal with a club option after making three impressive starts for the team at the end of the 2023 season. Given the track record of bullpen success the Yankees have had in recent years, you almost have to give them the benefit of the doubt when it comes to these kinds of acquisitions. It’s early, but we’re already seeing massive improvements from Weaver, whom the Yankees have transferred into a multi-inning relief role in this bullpen.

Taking over the Michael King role, the Yankees have seamlessly replaced a massive void in their bullpen, and Luke Weaver could be one of their most important arms in 2024.

The Data Behind Luke Weaver’s Breakout With the Yankees

MLB: Tampa Bay Rays at New York Yankees
Credit: John Jones-USA TODAY Sports

Pitching is a complex concept, but Matt Blake and Sam Briend seem to have these complex concepts down to a very simple science. While we do see some pitchers in the organization add sweeping breaking balls to improve their profiles, most of that development occurs at the Minor League level. For their more established arms, the Yankees seem to shift more towards maximizing their already-existing arsenal to improve what they’re comfortable throwing.

Take for example Clay Holmes, whom the Yankees emphasized the sinker with, a pitch that he already threw with the Pirates to some success. Optimizing an already-existing skillset can be difficult, but Matt Blake wields a mastery of both data and communication, making him a perfect coach for most pitchers. In the case of Luke Weaver, he was a fastball-cutter-changeup pitcher who experimented with curveballs, sliders, and sweepers all of last season.

When he came over to the Yankees, the team had him focus on the three key pitches he’s always had in his four-seamer, cutter, and changeup. These three pitches didn’t grade out particularly well according to Stuff+ last season, but seemingly small tweaks made over the winter have completely changed the way that Weaver can attack hitters right now.

Luke Weaver has seen massive improvements to two pitches specifically that rely on each other to be so effective, as his four-seamer and changeup are top-of-the-line offerings. Starting with his four-seamer here, the fastball is generating more vertical movement, and while his release height hasn’t changed much, the angle at which his four-seamer is approaching the plate has significantly improved.

In 2023 his four-seamer had a -4.6 degree Vertical Approach Angle, a metric that measures the angle at which a pitch crosses the plate, and this year he’s gotten that down to a -4.2. Why is that important? Well, the flatter the angle, the harder it is to hit a pitch located up in the zone, as batters struggle to get underneath it or on top of it for solid contact, and that’s been a huge difference-maker for Weaver thus far. A pitcher with a 1.41 HR/9 in his career before this season, we’ve seen that number quickly drop to just 0.82 as of the time I published this article.

With his four-seamer now significantly harder to hit than it had been in previous years, the other pitch that improved dramatically was his changeup, which is also affected by the four-seamer. On its own, the pitch is being thrown a bit harder with more vertical drop, both of which are positive changes, but because his four-seamer has more carry, it also means there’s a much greater difference in vertical movement.

MLB: New York Yankees at Arizona Diamondbacks
Credit: Mark J. Rebilas-USA TODAY Sports

Last season his four-seamer and changeup had 7.8 inches of vertical separation, but this season that number is at 13.9, causing batters to chase and whiff way more than they did against him last season. He ranked in the 15th Percentile in Chase Rate in 2023 (24.9%) but this season we’ve seen that number jump up to 33.9%, which is in the 90th Percentile for all pitchers.

With a 2.86 ERA and 3.23 FIP, Luke Weaver is becoming one of Aaron Boone’s most important weapons out of the bullpen, as the Yankees can call on him to hold leads across multiple innings. He got off to a slow start, but the Bronx Bombers are reaping the rewards of their excellent player development, and his emergence has been a huge part of this bullpen’s success early on.

The Yankees’ bullpen is first in ERA (2.30) and Win Probability Added (3.64) while also having the second-best HR/9 (0.50) and AVG (.192) as well. It’s one of the best relief units in the game, and Luke Weaver is one of the unsung heroes anchoring their excellent start to the 2024 campaign.

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