Overview
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Rating: 3.48 [distribution: poor (3-) 47.5%, excellent (8+) 0%]
2024 Stats:
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4 Games Started, 20 IP, 4.05 ERA, 4.23 FIP, 14 SO, 8 BB
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3 Games Reliever, 8.1 IP, 3.24 ERA, 4.99 FIP, 5 SO, 4 BB
2025 Stats
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1 Game Reliever, 3 IP, 9.00 ERA, 5.47 FIP, 1 SO, 3 BB
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2025 Earnings: Pre-arbitration
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2026 Status: On 40-man roster, Pre-arbitration eligible
Does he control the strike zone?
Is his control getting better? In 2024, in the minors and in the Majors, he showed a much improved walks per batter faced. In 2025, his control had a setback. My view is that it is fixable. Data from Baseball Reference.
Part of the improvement will be better control of the location of his slider. From the catcher’s view, sliders in the right side of the plate are more effective. Let’s talk about the reason when we talk about why he should be a starter.
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The following table shows his slider locations. Data from Baseball Savant.
In 2024 and 2025, his high contact rate, combined with his low strike rate, indicated a lesser control of the strike zone compared to other Diamondbacks starters. One caveat is that some of Diaz’s appearances were as a reliever. The following table compares him to other Diamondbacks starters.
If he can control the strike zone, he will belong in the Diamondback rotation.
Should he be a starter?
He pitched well against opposite handed hitters (left-handed hitters). That’s a foundation for being a starter.
His slider is a deathball. For that reason, this AZ Snake Pit article convinced me that he should be starter. The following quote says it best.
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“However when Diaz got to the bigs, lefties actually struggled against him more than righties did with a .255 BAA vs a .259 BAA in almost identical sample size. Thank you deathball! Because of this, I actually am more bullish on Diaz’s ability to stick in the rotation and he may actually have a much higher ceiling than many will realize. Opposing batters hit just .130 of Yilber Diaz’s deathball in 2024 with an almost 40% whiff rate! Not someone I really want to trade to be honest. Especially with his elite fastball.” — Jeff Irving, January 2025
But what about his poor start in 2025? Perhaps the deathball slider’s effectiveness depends on location, which was different than 2024 per the slider location table presented when we talked about controlling the strike zone.
Also, in that game he pitched fewer knuckle curves, making hitters more comfortable.
If he can command his slider, and if he pitches more knuckle curves, he will belong in the Diamondbacks rotation.
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What if he perfects a changeup?
In combination with his other pitches, it might make him “unhittable” per this AZ Snake Pit article. The following quote says it best.
“The changeup is still a work in progress. Diaz did not throw one last time out. Statcast has stripped it out of the data, re-classifying most of those pitches as slower fastballs and possibly a few as sliders. But it does exist. If perfected, it could be a weapon to go along with the slider, as it does produce similar vertical movement but basically the opposite horizontal movement.” — Preston Salisbury, July 2024
What is plan B?
He could be a reliever, perhaps a long reliever. On the internet, experts have written about why he could find success as a reliever.
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“Why did experts think that way [that his future was as a reliever]? His frame, his delivery and his lack of a third pitch.” — Makakilo, July 2024
If the Diamondbacks find better options to start games, then the plan B is for Yilber Diaz is to pitch as a reliever.
He is a success story.
Circumstances caused Yilber Diaz to step away from his dream of professional baseball. Two fortunate things helped him get back into baseball. After that, he ignored the wisdom of experts that he would be limited to the bullpen and he developed himself as a rotation pitcher per this AZ Snake Pit article.
My review of 2025 is very short.
In the Majors, one game in relief is a very small sample size. Therefore, no comment on 2025.
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Outlook.
The Diamondbacks need starting pitchers. As of 1 November, looking forward to opening day, the Diamondbacks have a core of three starting pitchers (Nelson, Pfaadt, and Rodriguez). There will be competition for the other two spots. Looking beyond opening day, injuries will likely require two additional starters to step into the rotation. Corbin Burnes will likely return late in the season (or possibly the following season). Therefore, an additional two spots will open later in the season.
Beyond the named pitchers, who else will pitch for the Diamondbacks? Per this AZ Snake Pit article, the strongest internal candidates follow.
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Christian Mena, Age 22.9.
Looking at their 2025 performances in AAA teams for the Diamondbacks and the Rangers, Diaz had the fastest average fastball velocity (95.3 MPH vs 94.1 and 93.3/92.7), Mena had the lowest wOBA (.337 vs .425 and .346/.395), and Drake had the lowest hard-hit percentage (30.0% for Rangers and 33.3% for Diamondbacks vs 43.1% and 44.7%). My view is that next season all three will be starters, whether in AAA or the Majors.
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Yilber Diaz will start games. He will reach and surpass his 2024 performance because better command (including his slider), better pitch mix, and possibly by adding a changeup.
