Is Yilber Diaz Ready?

Yilber Diaz (Photo by Chris Coduto/Getty Images) | Getty Images

Inspiration.

One of my favorite players was named minor league pitcher for the month of April.  Two accomplishments follow:

• Allowed no runs in 9 of his 10 appearances with the Aces.
• His 0.66 ERA ranked second among all Triple-A relievers

In July of 2024, I wrote about the secrets of his success. Circumstances made his future uncertain. He kept his dream alive and he worked hard (extraordinarily hard) to prepare, not knowing if he would ever get an opportunity.

In November of 2025, I wrote his player review.  The review considered three possible additions to the rotation (Yilber Diaz, Cristian Mena, and Kohl Drake), and it noted that Diaz had the fastest average fastball velocity (95.3 MPH vs 94.1 and 93.3/92.7).  Also, the review looked at his control of the strike zone, especially walks per batter faced.  In 2025 AAA, his 28.4% walks per batter faced was high.   Spoiler:  In 2026 through 8 May, his 9.0% walks per batter faced was an awesome improvement (as was his 11.1% in spring training).

Let’s look at reasons he is ready to be called up to the Majors.

Two tables tell the story.

This season in AAA he increased his control of the strike zone. The first table shows four of the five statistics were improved.  He increased the percentage of pitches in the strike zone, while causing batters to greatly increase their swing rate at pitches outside the strike zone (O-Swing), which likely reduced the batters’ success.  Data from FanGraphs, games through 8 May 2026.

This season, Diaz had more strikeouts than walks. Perhaps more remarkable were his 50% increase in whiffs and his 50% decrease in barrels per PA. The second table shows nine statistics that were improved. Data from Baseball Savant, games through 8 May.

My conclusion is that he clearly showed he is ready to be called up to the Majors.

What delayed his call up?

In spring training he improved his walk rate. On the other hand, his .33 hits per PA and his barrels per PA were too high.  That sample size was small (18 batters faced) and spring training did not show his full potential.

I’m happy that this season in AAA he overcame the small sample size. With 67 batters faced, he reduced his hits per PA by 50%, and he reduced his barrels per PA by 50%.

A second reason for delay is that the Diamondbacks are a top-5 team in shutdown performances by bullpen pitchers. They are only three shutdowns from being the best in the Majors. My view is that this metric is important for bullpen pitchers (other metrics show them closer to an average bullpen). One of the reasons he won pitcher of the month was that he allowed no runs in 9 of his 10 games. For that reason, he would be a great addition to the bullpen.

Summary.

Last season, Diaz walked too many batters. That issue is gone. This season in AAA his walk rate was much lower, and his ratio of strikeouts to walks was much higher. Perhaps more remarkable were his 50% increase in whiffs and his 50% decrease in barrels per PA.

This season in AAA he increased his control of the strike zone.  He increased the percentage of pitches in the strike zone, while causing batters to greatly increase their swing rate at pitches outside the strike zone, which reduced the batters’ success.

One of the reasons he won pitcher of the month was that he allowed no runs in 9 of his 10 games. For that reason, he would be a great addition to the Diamondbacks bullpen, which excels in shutdown performances.