How Alexis Díaz has become one of baseball’s best relievers

A year ago, Alexis Díaz burst on the scene straight from Double-A to become the Reds’ best reliever. It was a low bar, to be sure, but the young right-hander more than earned that title. Nationally, however, he wasn’t known as much more than Edwin Díaz’s little brother.

Fast forward to today, and the younger Díaz has joined his big bro in the upper echelon of MLB relievers.

Díaz has converted all 17 of his save attempts on the season. Only Rob Dibble has a longer streak to start a season (23) among Reds closers, doing so in 1991. But that’s merely the tip of the iceberg for Díaz. The 26-year-old has allowed only five runs and 10 hits in 28 innings this season. No qualified reliever has held opponents to a lower batting average (.108), expected batting average (.139), or slugging percentage (.161). He also ranks seventh in expected slugging percentage (.236) and 15th in ERA (1.61), and 11th in xERA (2.24).

Hitters don’t do much when they put the ball in play against Díaz, although that doesn’t happen often.

Already a strikeout artist last season (32.5 K%), Díaz has taken it up several notches in his second season by punching out nearly half the batters he’s faced (46.3%). Only one qualified reliever, Félix Bautista of the Orioles, has a higher strikeout rate than Díaz. Only two pitchers in baseball have seen a higher year-over-year jump in strikeout rate.

Díaz’s walk rate is still high (11.1%, 19th percentile), but it’s down from last year (12.9%, 2nd percentile). Pitchers who thrive with a troublesome walk rate usually do so because they’re missing a lot of bats, which Díaz does often. As a result, the fielding-independent metrics — which try to isolate factors a pitcher can control — still love what Díaz is doing in 2023.

Díaz ranks sixth among qualified relievers in FIP (1.78), fourth in xFIP (2.33), and first in SIERA (2.01).

But how exactly does Díaz do it? What has made him special enough to jump from Double-A to the majors and start dominating the world’s best hitters in short order? And what has he done to take a leap forward in his second season? Let’s take a closer look.

The pitch mix

Díaz is a classic two-pitch reliever, throwing only a four-seam fastball and slider. His average fastball velocity (94.7 mph) is above average (69th percentile) but is down by 1 mph this year. His spin rate is elite (97th percentile), although is also lower than last year by about 70 rpm. But those decreases haven’t affected his results much.

Díaz likes to pepper the top of the zone with his fastball, and hitters typically can’t do anything with it. Opponents are hitting .135 (.177 xBA) and slugging .162 (.304 xSLG) with a .261 wOBA (.319 xwOBA) against his four-seamer in 2023. Here are the league averages against four-seam fastballs:

  • .261 BA (.259 xBA)
  • .454 SLG (.467 xSLG)
  • .352 wOBA (.358 xwOBA)

Díaz also has a 34.4% whiff rate on his heater, well above league average (22.0%) and up from his 2022 mark (31.1%). Among relievers with at least 90 swings against their fastballs, only seven have a higher whiff rate.

Starting midway through last season, Díaz began to throw his slider about 45% of the time instead of 30% of the time.

The shift in pitch mix has unlocked a new level for Díaz, and it’s when his rise from “good” to “elite” began. Since the All-Star break last season, Díaz is tied for the lead among all relievers in fWAR (2.2).

Díaz’s slider doesn’t get remarkable movement, but that’s not the most important trait for a breaking pitch. What’s more important is how well it plays off his electric fastball. Only five pitchers have sliders with a better run value — which measures the run impact of a pitch based on the runners on base, outs, ball and strike count. Opponents are hitting .089 (.113 xBA) and slugging .161 (.189 xSLG) with a .146 wOBA and .169 xwOBA against his slider. Díaz has a 47.4% whiff rate with the pitch, up from 45.0% last year and significantly better than league average (33.6%).

By throwing more sliders, Díaz has seen the biggest gains against left-handed batters.

When he was throwing the fastball nearly 70% of the time, Díaz made it a bit easier for hitters to guess what was coming. Díaz’s fastball gets a lot of whiffs, but the slider is his best swing-and-miss offering. By throwing each pitch about half the time this season, he’s keeping hitters guessing and getting more whiffs and strikeouts overall.

Díaz has also shifted his approach against lefties, throwing fastballs up and away instead of up and inside, while getting more comfortable throwing sliders on both sides of the plate. You can see the before and after in the GIF below:

You can see how Díaz’s pitch mix gives lefties fits in this at-bat against Kyle Schwarber from earlier this season. Díaz starts Schwarber with a high fastball, eliciting a big swing and miss.

He follows up with back-to-back sliders, getting ahead 1-2 in the count. Schwarber expects another high fastball as Díaz seeks a strikeout. But the young right-hander fools the veteran hitter, freezing Schwarber with a slider for strike three.


A truly unique delivery

Plenty of pitchers throw hard and get good spin, but Díaz’s has some tricks up his sleeve to get even more out of his arsenal. The first is his unorthodox delivery to the plate. Here’s a look at the average release points of every right-handed pitcher who has thrown at least 200 pitches this season:

Díaz is the red dot. While he’s not an enormous outlier, he does throw from a fairly unique arm angle. Starting from the first-base side of the rubber, he throws from a wider, much lower arm slot than most pitchers. This gives hitters a unusual and uncomfortable look, creating deception for Díaz.

And he has continued to drop his arm angle lower. Although he has settled on a slightly more consistent release point this year, he continues to see how low he can go.

Here’s where Díaz really starts to separate himself from other pitchers: he combines a deceptive arm angle with elite extension. No pitcher in baseball gets closer to home plate before releasing the ball. With an average extension of 7.7 feet — up from 7.5 feet last year, by the way — Díaz releases the ball about 52.8 feet from home plate, 1.3 feet closer than the typical MLB pitcher. That equates to less reaction time for the hitter and makes Díaz seem to throw faster than the radar gun indicates.

While the actual average velocity of Díaz’s fastball is 94.7 mph, the average perceived velocity from the hitter’s standpoint is 97.0 mph. Only two pitchers in baseball have a larger gap between actual and perceived fastball velocity, likely due to small differences in release point.

The actual velocity of his slider is 87.4 mph, but the perceived velocity is 89.8 mph. That’s the widest gap between actual and perceived velocity of any slider in the game.

And that’s still not the end of what makes Díaz a unicorn. He combines his low arm slot and extension to achieve a high vertical approach angle (VAA). This is the angle at which a pitch is traveling, relative to the ground, when it crosses the front of home plate. Because the mound is elevated, almost all pitches have a negative VAA. Every pitch is impacted by its VAA, but it’s most important for four-seam fastballs. A fastball with a “flat” VAA will not drop as much as the average heater, leading to more whiffs and pop-ups when thrown up in the zone. It’s similar to a fastball with high spin and spin efficiency.

In the case of Díaz, a “high” VAA just means his fastball crosses the plate closer to zero degrees than the average heater. Because Díaz gets a lot of extension and has a low arm slot, he doesn’t need to create as steep of downward angle to get the ball in the strike zone, especially when he throws it up. Díaz’s average fastball has a VAA of -3.5 degrees, which tied with Mariners reliever Paul Sewald for the highest in baseball among pitchers who’ve thrown at least 200 fastballs. The league average four-seamer has a VAA of -4.8 degrees.

We know Díaz gets whiffs by throwing high heat, but his VAA also helps him get poor contact when hitters get a piece of the ball. Díaz gives up a lot of flyballs, but they’re rarely hit at a launch angle where they can do damage. The right-hander has a 16.3% pop-up rate this season, ranking 13th out of 486 pitchers with at least 25 batted balls against them — hence, another big reason why he ranks in the 97th percentile or better in xBA, xSLG, and xERA.

Final Thoughts

As you can see, there simply aren’t any other pitchers like Díaz. He has some skills that make him naturally deceptive and tough to hit, such as his velocity, extension, arm angle, and VAA. Now that he’s throwing his fastball and slider at a nearly even split, Díaz is leaving hitters with even less of a clue about what’s coming. Righties have never been able to hit him, and now he’s finding the same success against lefties.

As a result, Díaz has quickly established himself as one of the best relievers in baseball. And next month, he should join his big brother as the second All-Star reliever in the Díaz family.

Featured photo: Joe Robbins/Icon Sportswire

Matt Wilkes

Matt Wilkes got hooked on Reds baseball after attending his first game in Cinergy Field at 6 years old, and he hasn’t looked back. As a kid, he was often found imitating his favorite players — Ken Griffey Jr., Adam Dunn, Sean Casey, and Austin Kearns — in the backyard. When he finally went inside, he was leading the Reds to 162-0 seasons in MVP Baseball 2005 or keeping stats for whatever game was on TV. He started writing about baseball in 2014 and has become fascinated by analytics and all the new data in the game. Matt is also a graduate of The Ohio State University and currently lives in Chicago. Follow him on Twitter at @_MattWilkes.