Height: 6'3" Weight: 250 lbs.
Bats/Throws: Right / Right
Born: 01/18/1994 in Cabrera, Dominican Republic
Offseason home: Cabrera, Dominican Republic
Nickname: Big Diesel
Family: Daughter Diannelys
MLB Debut: 06/06/2018
Acquired: In trade from the Tampa Bay Rays, for J.T. Chargois and Austin Shenton (07/29/2021)
Free Agent after: 2024 season
Looking at Castillo's season numbers, you might think he's been a bust in the Mariner ’pen. But that isn't the case—his high ERA and WHIP marks are mostly due to three appearances over one week of the season, back in early May. They were brutal outings for "Big Diesel," totaling nine earned runs allowed over just one full inning of work, but whatever may have been bothering him either ran its course or got corrected. Remove those three games from his ledger and you get an ERA of 2.00 even and a WHIP of 0.9444. Since the bad week, Castillo has allowed runs to score just once, in a game in Boston wherein it had no effect on the outcome of the game (the M's were losing that one anyway). With injuries still hobbling the relief corps and Drew Steckenrider becoming this year's bullpen punching bag before getting demoted, Diego is among the more reliable closing options for manager Scott Servais, particularly if he's brought in with no one on base and doesn't need to worry about his hard slider getting by the catcher.
Fans were not pleased when reliever Kendall Graveman was traded to Houston prior to last year's trade deadline, but a couple of days later general manager Jerry Dipoto swung another deal to fill the closer vacancy, landing Castillo from the Tampa Bay Rays. Castillo's Mariner tenure didn't begin well—he surrendered a walkoff home run to blow his first save opportunity—and for whatever reason manager Scott Servais didn't choose to use him in the role he was acquired to fill. The Dominican righty only logged two saves in his Mariner tenure last year (to go with the 14 he collected as a Ray) with most of the opportunities going to Paul Sewald and Drew Steckenrider. He doesn't figure to be given first crack at the ninth inning this year either, as veteran Ken Giles is expected to close, at least at first. Whether he sets up or closes, Castillo should be formidable. Over four Major League seasons, he's posted a WHIP just a hair above 1.000, walking roughly one batter every three innings while striking out three of ten batters faced.