On Wednesday morning, Ken Rosenthal tweeted Josh Hader's stats so far this season:
And…60 fastballs out of 62. https://t.co/gXiyGyCLJp— Ken Rosenthal (@Ken_Rosenthal) April 3, 2019
The second tweet caught my attention. A 97% four-seam fastball rate is more or less unheard of, even through a small sample. In 2018, Hader threw his fastball 77% of the time, and went to his slider 21% of the time. This year, he's thrown 60 fastballs and one slider. Even Kenley Jansen, one of the game's best relievers who relies primarily on one pitch, threw his cutter just 80.4% of the time last season.
This lopsided pitch mix could be cause for concern in another reliever, but Hader his still been as dominant as ever. So far, he has a 52.9% K-BB% compared to 36.9% in 2018, and a 40.3% swinging-strike rate, more than double last year. Here he is throwing an immaculate inning with nine straight fastballs:
Hader seems to be doing this intentionally. So, what could be the reason for Hader's fastball-only repertoire in his first several appearances? Let's examine a few, from least to most likely.
Yasmani Grandal isn't comfortable catching his slider.
While Grandal has been behind the plate for all of Hader's pitches, the idea that he is behind this change seems unlikely. The Brewers surely expect Grandal to be able to catch their best reliever's full repertoire, and Grandal rates as one of the best defensive catchers (though he seems to be an average blocker).
Specific batter matchups dictated it.
Hader faced Paul Goldschmidt, Paul DeJong, Marcell Ozuna, Yadier Molina, Dexter Fowler (twice), Jose Martinez, Tyler O'Neil, Yairo Munoz, Curt Casali, Scott Schebler, Joey Votto, and Yasiel Puig before throwing anything but his fastball. He then threw a changeup and slider (among fastballs) to Jose Iglesias. After that, Tucker Barnhart, Jose Peraza, and Kyle Farmer only saw his fastball. Hader has faced a wide array of hitters with varying approaches and skill levels, and it is unlikely that they all categorically struggle against fastballs.
The Brewers medical staff thinks that throwing the slider puts more stress on Hader's arm, and relying on the fastball will keep him fresher throughout the season.
While Hader's average velocity stayed relatively consistent throughout the 2018 season, his spin rate peaked in April. It is possible that Hader feels the slider is harder on his arm and that he would rather save the pitch for later in the season. However, it seems more likely that Hader's usage early in the season contributed to the observed declines in spin rate, which corrected themselves in the postseason regardless.
Shorter outings allow Hader's fastball to play up a bit.
Hader's four seamer averaged 2043 RPM last year. This year (albeit in a limited sample with a fresh Hader), it's all the way up to 2302 RPM, closer to his average slider from 2018 than his fastball from the same year. Hader has similarly experienced a slight uptick in velocity on his fastball of 0.7 MPH. So far this season, Hader has been used in a more limited role. Just like many starters’ receive a velocity bump when they move to a one-inning bullpen role, it seems possible that Hader could receive the same bump.
Both this and the previous explanation could explain why Hader feels comfortable throwing his fastball as often as he is, but they don't explain why he has only thrown one slider. Which brings us to explanation five…
Hader hasn't needed his slider yet.
Hader was briefly asked about his early reliance on his fastball by the Milwaukee Journal Sentinel's Tom Haudricourt:
Josh Hader talks about relying on his fastball and Christian Yelich coming through again. pic.twitter.com/zRhldtp66L— Tom (@Haudricourt) April 2, 2019
His reasoning is pretty simple: hitters can't seem to deal with his fastball, so why throw anything else? When it stops working, he'll bring out the slider, but he's confident enough in the fastball to throw it almost exclusively until hitters give him a reason not to.
It is extraordinarily unlikely that Hader will keep up this rate of fastball usage for the entire season, but even this small stretch to start the season is impressive. Often, pitchers need to add another pitch to keep up with competition, but Hader, one of baseball's best relievers, is showing that he can subtract a pitch and still get results.
Pitch data from Fangraphs. Video obtained through Baseball Savant's new video search function.