SURPRISE, Ariz. -- José Castillo's brutal run of injury luck took another agonizing turn this week.

The Padres’ left-hander was diagnosed with a UCL tear, manager Jayce Tingler said on Thursday. Castillo will undergo Tommy John surgery and miss the entire 2021 season.

Castillo exited a simulated game with forearm tightness on Tuesday after six pitches, prompting further exams.

After his breakthrough 2018 campaign, Castillo seemed poised to turn himself into an integral piece in the Padres' bullpen. But he has pitched in just one game over the past two years because of an assortment of ailments -- a left lat strain, a left flexor strain and a left middle-finger injury.

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"It's obviously crushing," Tingler said. "Everything he's been through, obviously, with battling injuries and the way he's worked to come back, it's frustrating, crushing for him."

In 39 career innings, Castillo owns a 3.23 ERA with 54 strikeouts. He boasts an electric fastball/slider combo that made some team officials wonder whether he was destined for closer duties.

Injuries have prevented the 25-year-old Castillo from realizing his huge upside. Tingler expressed optimism that -- after three seasons’ worth of stops and starts -- Castillo at least could have a fresh start when he returns in 2022.

Without Castillo, the Padres still boast a deep bullpen mix. Left-hander Matt Strahm is being built up slowly after offseason surgery on his right knee. The Padres won't rush him back, and a return for Opening Day seems doubtful.

If anything, Castillo's absence might further solidify bullpen spots for lefties Tim Hill and Adrian Morejon. As things stand, the Padres still have eight or nine relievers competing for, at most, four available bullpen places.

Neither Castillo nor right-hander Javy Guerra is among that group any longer. Tingler also revealed that Guerra will be shut down for two to four weeks with a Grade 1 UCL sprain in his right elbow. Guerra, who was doubtful to crack the Opening Day roster, will be reevaluated in late March.