On December 26, 2024, the Rockies signed right-handed reliever, Diego Castillo (not utility-man Diego A. Castillo, older brother of Rockies prospect Juan Castillo) to a minor league contract to try and improve their bullpen for the 2025 season.

Castillo has appeared in MLB every season since 2018, and from 2018-2022, Castillo was one of the best relievers in baseball where he picked up 3.12 ERA across 259.2 IP, while also picking up 35 saves.

Castillo’s Career

The Tampa Bay Rays signed Diego Castillo as an international free agent out of the Dominican Republic in March of 2014. From there, he steadily progressed through the Rays system and grew his prospect status with his high octane arm. At the start of the 2018 season with Triple-A Durham, Castillo put up an impressive 1.03 ERA in 26.1 IP, granting him an inevitable MLB call-up. 

As a new member of the Rays bullpen, he quickly became a contributing member — even starting 11 games as an opener/limited starter — ending his rookie season with a 3.18 ERA across 56.2 innings pitched. Castillo stayed with the Rays until they traded him to the Seattle Mariners in exchange for J.T. Chargois and Austin Shenton in 2021. That season he shared 16 saves with Tampa Bay and Seattle, still his career high. 

Also worth mentioning is Castillos’s excellence in the postseason for both teams as well. He threw 18.1 innings across three separate playoffs runs in 2019, 2020, and 2022. All together, he combined for a 0.98 ERA, which would be the fourth lowest in the playoffs from 2019-2022. 

In 2022, Castillo started struggling with walks, but it wasn’t enough to hurt him at that time walking an acceptable 3.6 per nine innings pitched. That was just the start of a steep decrease in command. He started 2023 with a 7.3 BB/9 and after only 8 games he was put on waivers, spending the rest of the year in the minors.

Castillo elected free agency at the end of the season and then signed a minor league contract with the Texas Rangers. In Spring Training he struggled once again with a 4.70 ERA, leading him to be released before the season started.

From there, the Minnesota Twins decided to give him an opportunity to pitch and Castillo spent 2024 with them with sporadic appearances at the MLB level. However, in 40 Triple-A games he had a 5.59 ERA, struggling once again. As he elected free agency again at the end of the season, he now signed with the Colorado Rockies.

So why did the Rockies sign him?

Castillo’s Arsenal

The 6-foot-3, 268 pound Diego Castillo has a high-energy delivery, and throws the ball with his entire body. It comes off weird to the eye, but it’s efficient and powerful and has worked well for Castillo. His pitch repertoire includes a mid to high 90’s sinker, an above average slider, a 4-seam fastball, and an occasional changeup. 

Sinker

A pitch known for significant arm-side run is Castillos sinker, averaging 15.7 inches of horizontal  movement, and 4.2 inches of rise. The sinker has contributed greatly to Castillo’s career 51.2 groundball percentage. The ability to keep the ball on the ground is one of the more important attributes for pitching at Coors Field; any advantage you can garner for the most hitter-friendly ballpark should be sought after.

In terms of Run Value (the higher the number is, the better it has performed), Castillo’s sinker hasn’t been great, compiling a -12 Run Value throughout his career. However, the sinker does its job extremely well, getting ground balls, and setting up his other pitches, though it ends up hit hard quite a bit, too. 

Slider

Despite Castillo’s struggles in 2023 and 2024, his slider has always been well above average. Throughout Castillo’s career, opponents have hit only .143 against his slider. Making it even more impressive, he has thrown his slider over 60% of the time in his career. Throwing it at a high rate with such good results has cemented it as one of the better pitches in baseball since 2018. Generic sliders, such as Castillo’s, have always performed respectably at altitude, making it a pitch that projects well in Colorado. Since 2018, Castillo’s slider has a 35 Run Value; in 2021 he was tied for first for the best Run Value at 15 (tying with Clayton Kershaw).

4-Seam Fastball + Changeup

Early in Castillo’s career he threw his 4-seam fastball around 10% of the time, though it was averaging 99 MPH then. Ever since, he’s started throwing it less and less. Now it is more so used as a pitch to catch batters off guard with a similar velocity as his sinker. In Colorado, we could start to see Castillo use the pitch more as 4-seams typically have more success at altitude than sinkers. As for his changeup, he only started throwing it in 2023 (which was when he started getting poor results), so there isn’t much sample size other than it being a back-pocket option. His changeup has 9.8 inches of horizontal run and 4.2 inches of rise. 

Potential that Diego Castillo Brings:

The most encouraging sign of a possible career resurgence for Diego Castillo is an encouraging increase of pitch velocity last year after seeing a concerning dip in 2023. In 2023, his average fastball velocity was 93.5, the lowest of his career (his age 29 season).

Only just a year later, he was throwing the fastest since his velocity started to decrease in 2021. His fastball averaged 95.9 MPH, and his sinker 95.2 MPH. If he continues to struggle with command, his pitch velocity will be able to pass hitters and induce whiffs, though he’ll need to command his stuff more to be successful.

When Castillo was traded to Seattle, his arm angle went down by 7° (from 34° to 27°) which could be a reason for his eventual loss of control. Though in 2024, it went back up to 33° which is similar to what it was in his 2020 campaign. 

With both his velocity/pitch peripherals, and arm angle back to what it was before he lost his command, it gives the Rockies hope that the command will come back naturally. Castillo can turn into one of the more important relievers in the Rockies bullpen in 2024, and can finally turn around his career. 

Final Thoughts

As there has been no signs of command improvement the last couple of seasons for Castillo, there is no risk with signing him to a minor league contract. Effectively, Castillo could be your typical diamond in the rough player that was signed for cheap. The upside is far more valuable than not signing him.

Castillo looks to join Jefry Yan, Jimmy Herget, Tyler Burch, and Jack O’Loughlin as fun, experimental Arm Barn candidates for 2025, which just might be one of the more diverse and effective bullpens in recent years for Colorado.