Mason Miller's Unholy Evolution Mirrors That of Edwin Diaz

[N.B. Lindberg]

Through the first week and a half of the season, the best pitcher in baseball has been San Diego Padres closer Mason Miller. Over 6 1/3 innings, he has allowed exactly zero runs, struck out 16, walked one, and allowed a single solitary single. He leads all relievers in FIP (-1.43), fWAR (0.5), and strikeout percentage (76.2%) — a negative FIP is truly demonic. You basically cannot pitch better than Miller has, and like so many elite relievers before him, the man armed with the league’s fastest average fastball velocity (101.2 MPH) is dominating with... his slider?

Yes, Mason Miller, he who burst onto the scene in 2024 with the then-Oakland Athletics by throwing straight 100 MPH gas past MLB hitters, has bent his arsenal away from the pitch that made him famous and has never been better. 

Thus far, he has thrown his slider 52.5% of the time, followed by his fastball at 42.5%, and his nascent changeup, which deserves its own look, clocks in at 95.5 MPH and 5.0%. Compared to 2024, when he reared back for a fastball on 63.1% of his pitches, Miller now looks like a junker-baller, but it has unequivocally been a successful evolution. 

The question isn’t if Miller has made the right choice; it’s why he decided to even entertain (relatively) shelving such an elite fastball. And there’s a pitcher just north on the interstate who made the same fateful decision. 

Before Edwin Diaz became a national sensation for making trumpets cool again, he was your quintessential fireballing reliever. Between 2016 and 2021, Diaz used his upper-90s fastball at least 60% of the time to solidify himself as one of the game’s top closers. His high point came in 2018, when he threw his fastball 62.2% of the time, struck out 15.22 batters per nine innings, and racked up 3.5 fWAR, the seventh-highest reliever total of the 21st century. Diaz had a formula that not only worked but arguably made him the best.

Then in 2022, Diaz suddenly flipped his fastball usage to 41.9% and his slider usage to 58.1% and posted another all-time season. He rode his slider to a 1.31 ERA, 17.13 K/9, and a fWAR of 3.0. In the following seasons, he has eased his slider usage a tad, but he still throws both pitches about half the time. Now, Diaz’s evolution would make sense if his fastball velocity fell off a cliff, but his fastball averaged 97.3 MPH in 2018 and 97.2 MPH in 2025. What caused Diaz to stray off the straight and narrow is the same force that looks to have pushed Miller: throwing strikes. 

Pitching is fundamentally about strikes, and you can get them one of two ways. You throw the ball in the strike zone, and the umpire calls it, or you get a batter to swing and miss. The best pitchers do both, as do the best pitches. If you rack up a bunch of called and swinging strikes, it’s pretty hard not to have success. 

Edwin Diaz and Mason Miller, in particular, might have elite fastball velocity, but that doesn’t mean it’s their best pitch. Miller’s fastball has never had outlier horizontal or vertical movement, which means it relies mainly on velocity to be effective, and Diaz, despite not losing velocity over the years, has lost vertical movement, which has hurt the pitch’s effectiveness. On top of that, neither has been particularly accurate with their fastballs. When you add it all up, their heaters were effectively wild, which stood in stark contrast to what their sliders became. 

A decent proxy for what pitch a pitcher should lead with is a little metric I call, “Zone Percentage + Outside Zone Swing and Miss Percentage.” While I need to find a catchier, shorter name, the metric’s simplicity is why it works. Pitchers want to throw strikes, and what better way to do that than to throw it in the strike zone? Also, when pitchers don’t throw it in the zone, they want someone to swing and miss because that’s still a strike. Looking at Diaz’s four-seamer and slider figures, it’s pretty obvious why he has ramped up his slider usage in recent seasons.

*Data reflective of updated stats as of April 9, 2026

Edwin Diaz Season	Pitch	Pitches	O-Swing%	O-Contact%	Zone%	Zone% + O-Swing Strike
2017	Fourseam (FA)	772	27.50%	64.60%	54.80%	64.54%
2017	Slider (SL)	354	44.10%	30.90%	39.80%	70.27%
2018	Fourseam (FA)	727	24.60%	60.30%	59.10%	68.87%
2018	Slider (SL)	436	40.10%	22.40%	38.80%	69.92%
2019	Fourseam (FA)	706	28.60%	66.30%	54.40%	64.04%
2019	Slider (SL)	363	37.60%	24.70%	46.60%	74.91%
2020	Fourseam (FA)	294	23.10%	51.60%	54.40%	65.58%
2020	Slider (SL)	182	34.70%	25.70%	44.50%	70.28%
2021	Fourseam (FA)	615	21.40%	61.30%	52.80%	61.08%
2021	Slider (SL)	378	41.40%	24.40%	47.60%	78.90%
2022	Fourseam (FA)	389	26.20%	53.30%	55.80%	68.04%
2022	Slider (SL)	539	48.80%	23.60%	43.80%	81.08%
2024	Fourseam (FA)	475	18.80%	52.40%	52.80%	61.75%
2024	Slider (SL)	437	37.50%	41.70%	48.70%	70.56%
2025	Fourseam (FA)	560	25.10%	44.40%	55.20%	69.16%
2025	Slider (SL)	505	34.20%	35.60%	47.90%	69.92%

Diaz has never thrown his fastball in the zone that often, and fastballs just never garner nearly as many swings out of the zone. Meanwhile, his slider has always enticed an incredible amount of swings out of the zone, while being nearly unhittable. Unsurprisingly, his best season as a primary fastball pitcher was 2018, when he posted the best Zone% of his career with the pitch, but it’s hard to lead with a fastball, even a great one, that you can’t consistently throw for a strike. And the same trend follows Miller. 

Mason Miller Season	Pitch	Pitches	O-Swing%	O-Contact%	Zone%	Zone% + O-Swing Strike
2023	Fourseam (FA)	338	28.80%	69.00%	56.80%	65.73%
2023	Slider (SL)	145	25.30%	38.10%	42.80%	58.46%
2024	Fourseam (FA)	646	37.60%	52.40%	56.80%	74.70%
2024	Slider (SL)	358	37.80%	27.00%	45.30%	72.89%
2025	Fourseam (FA)	507	31.20%	48.10%	50.10%	66.29%
2025	Slider (SL)	443	42.50%	16.20%	44.20%	79.82%
2026	Fourseam (FA)	26	35.70%	80.00%	46.20%	53.34%
2026	Slider (SL)	31	57.10%	12.50%	54.80%	104.76%

It’s incredibly early, but what Miller is doing with his slider is basically a cheat code. Major league hitters are chasing it out of the zone over half the time and are coming up empty at a 87.5% rate. To make matters worse, for hitters, he’s throwing his slider in the zone at a 54.8% clip, where he’s allowing a 40% contact rate. These numbers should all regress to the realm of mortals eventually, but that shouldn’t quell any of its usage, especially because his fastball command has continued to back up with a career low 46.2% zone rate. 

For as excellent as Miller has been, there’s a chance he has another gear. In 2024, Miller’s fastball had a 70.3% contact rate in the zone but that figure rose to 75.3% in 2025, while also finding itself in the zone less often. Since it was generating fewer strikes, it makes sense he would lean less on his fastball in 2025, but there’s a chance he could rediscover his 2024 fastball mojo. 

In 2026, his four-seamer again has an in-zone contact rate of 70%, in line with his 2024 figure, and that’s despite hitters clearly sitting on the pitch. Batters have swung at 83.3% of the in-zone fastballs they’ve faced, which far exceeds his career average of 71.2%. At a certain point, hitters will have to stop sitting on his fastball if they want any hope of handling the slider. And when that happens, don’t be surprised if his fastball becomes a major weapon again. Miller might not be pitching off his four-seamer anymore, but hitters are certainly treating it like he is. 

The final evolution for Mason Miller is to find the harmonic 50/50 fastball-slider split. The former was such a standout pitch that hitters sold out to hit it, only to be destroyed by the latter. When Diaz did that in 2022, he had a season for the ages, but eventually batters began to lay off his slider, and his fastball usage rebounded. The truly terrifying thing is Miller’s stuff is even better than Diaz’s was, and if he ever figures out a change-up... well, let’s not scare the kids.

 

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