Why Diego Castillo is Still a Dominant Back End Reliever

The Seattle Mariners made some major waves at the trade deadline when they traded reliever Kendall Graveman for Abraham Toro. The clubhouse was furious and there was lots of fan uproar as well. That was followed up when the Mariners made a separate trade for then Tampa Bay Rays reliever Diego Castillo, who had been their closer or 8th inning man for a few years and for multiple playoff runs. He got a bad rap on the Mariners down the stretch and is a much better reliever than people think.

Diego Castillo has been one of the better relievers in baseball and was having another good year in Tampa Bay before coming over to Seattle. For fans of ERA, he had an ERA of 2.72 with the Rays and an ERA of 2.86 for the Mariners. That is a very tiny difference, especially for a reliever since they have far fewer innings than a starting pitcher. His ERA+, a league-adjusted stat where 100 is league average, was one point better in Seattle. 147 compared to 146 in Tampa.

Diego Castillo is a great reliever and will be important to the Mariners

The thing that fans jumped onto early with Castillo was saying that he is like Fernando Rodney and wild and it is just an experience. He did walk a few batters more than he normally does in his first few appearances, and that is what gave him his bad rap. His strikeout to walk ratio did drop a bit, it was still in the range of Castillo’s career average. He still also struck out 10.6 per nine. This matched his 2019 season, probably his worst year.

Diego Castillo is still a very good reliever. He was worth 0.7 WAR for the Rays in 36 innings, and in 22 innings for the Mariners, he was worth 0.4 WAR. By comparison, Yohan Ramirez was worth 0.3 WAR and this was in just over 27 innings, so Castillo was still very valuable. It is also worth noting, that Castillo was traded from one corner of the country to another. It would not be easy for any human to adjust to that right away.

The underlying metrics are still quite good for Diego Castillo. Opponents actually had a worse batting average against him in Seattle. They hit an even .200 against him on the Rays and only hit .184 against him on the Mariners. The only number that went up that was maybe a little concerning was his FIP, which rose from 3.17 to 4.67. That is not a great number but it was only over 22 innings. That is such a small sample size that it is not a big red flag.

His advanced numbers are all still very elite for the whole 2021 season. His xERA (Expected ERA) was in the 93rd percentile, as was his K rate. His xBA (Expected batting average against) was in the 92nd percentile. Then there is his chase Rate which came through in the 94th percentile.

All of his numbers listed above are in some of the darkest red colors on Baseball Savant. Diego Castillo gets lots of swings and misses on the wipeout slider that he throws 66% of the time. He also throws a sinking fastball to contrast his slider. He occasionally throws a straighter four-seam fastball, but he only threw that 38 times last year, making up 4.1% of his pitches.

The Mariners have a back-end stud in Diego Castillo who gets strikeouts at an incredible rate. Paul Sewald struck out batters at a 39% rate, which is an insanely high number, unlikely to be repeated. With Tampa Bay, Castillo was striking out about 33% of batters in 2021, and still had a 29% strikeout rate with the Mariners. If Castillo is striking out about 30% of batters in 2022 while continuing to have a solid walk rate, the Mariners will have one of the best late-inning relievers in baseball.