What happened to Seattle Mariners reliever Diego Castillo?

August 18, 2023

It wasn’t long ago Diego Castillo dominated in back-to-back seasons (2018, 2019) for the Tampa Bay Rays and then again in 2021 for both the Seattle Mariners and Tampa Bay Rays (he threw just 21.2 innings in 2020).

Unfortunately, since then, it has been a slow decline for Castillo and he’s found himself in Triple-A ball for all of 2023, save for 8.2 innings for Seattle at the beginning of the year before being demoted. It hasn’t looked good there either, in 36.2 IP in the minors this year, he’s pitched to a 6.38 ERA with a 2.02 WHIP with 29 walks issued and just 34 strikeouts to go with it.

Castillo is just 29-years-old. What could’ve caused him to go from consistent big league high-leverage reliever to struggling to get outs in the minors? Let’s see what the numbers tell us.

Starting with the slider, Diego’s most thrown pitch, and what is supposed to be the put-away pitch.

YearAverage Velocity (MPH)Horizontal Break (inches)Whiff%

The slider has gone from a pitch that had 56% more horizontal break than other sliders in the league with similar velocity and extension metrics in 2018, to having -92% horizontal break compared to other sliders in the same category in 2023. To put it in simple terms, the breaking ball isn’t breaking, and it’s coming at hitters roughly 2.5 MPH slower than it was during the 2018 and 2019 campaigns. The result? Opposing batters are making hard contact on the pitch. In 2018 batters only could muster a 25% hardhit% against the pitch. In 2023? 45.5%.

Next up, the sinker.

YearVelocity (MPH)Inches of DropWhiff%

We can see the slow decrease in velocity, but Diego has managed to combine it with an increase in movement. The result has been more whiffs, but unfortunately for the righty it hasn’t warranted better results. Batters still hold an xBA (expected batting average) of .287 against the pitch in 2023 and an eye-popping .598 xSLG. This is a pitch Diego has decided use less as of late. In 2018 he threw it 42.1% of the time. In 2019, 44.8. In 2023, just 21.6.

The four-seam fastball:

YearVelocity (MPH)Vertical Movement % Break vs AvgWhiff%

We see the velocity dip once again. The key here is that despite the velocity dip, the fastball has remained just as flat as when Castillo was throwing it almost 99 mph every outing. 98-100mph fastballs don’t typically have tons of movement, and don’t typically need it. But 92-95 mph fastballs, do. Which is why we see the vertical movement % vs what the league average is continually get worse as the velocity decreases. Ignore the 2023 whiff% here, as he only threw the pitch a total of 18 times in 2023 in the big leagues. Diego clearly began to lose control of the fastball beginning in 2021, shown here:

As a result, the right-handed reliever attempted to develop a changeup over the summer of 2022 heading into 2023. However, he threw it just 16 times in the majors and opponents hit .333 on it.

The underlying issue here seems to be the change in arm slot over time. Going from an average vertical release of 6.3 feet in 2018 to 5.6 feet in 2023 which could certainly help explain the lack of vertical movement on the fastball as the velocity decreased and the elbow gradually came down. Injuries in 2019, 2021 and 2022 to the shoulder could cause Castillo discomfort when releasing the ball that high, forcing him to bring the release point down. Below, we can see the change clearly.

On the left, Castillo throwing a 94 MPH four-seam fastball on April 11, 2023. On the right, he’s throwing another four-seam fastball on October 8, 2019 vs the Astros in the ALDS, this time 98 MPH.

It has been an unfortunate turn of events for what was one of the most electric relievers in MLB for a few years. Unfortunately, barring a miraculous recovery from the shoulder, Castillo is going to have to reinvent himself — or somehow find the previous version — to get back to the majors and find success.

Casey Bellon

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