DUNEDIN, Fla. -- As Luis Severino dropped into a chair in the Minute Maid Park press conference room last October, the right-hander expressed confidence that he would be pitching the next evening, believing that his Yankees would push the American League Championship Series against the Astros to a seventh game.

That did not transpire, and now Severino is not expected to make his next trip to a big league mound until sometime in 2021. Severino has been diagnosed with a partially torn ulnar collateral ligament, an injury that will require Tommy John surgery, Yankees general manager Brian Cashman announced on Tuesday. Dr. David Altchek will perform the procedure Thursday at the Hospital for Special Surgery in New York.

“Sevy has acknowledged and agreed the surgery is necessary,” Cashman said during the Yanks’ 4-1 Grapefruit League victory over the Blue Jays.

Severino’s issues date back to that ALCS, when he mentioned discomfort in his right forearm while receiving routine treatment following a Game 3 start against Houston. Although Severino indeed would have been on the mound if the Yankees had forced Game 7, he continued to experience discomfort during the offseason, especially when throwing his changeup.

Two trips from the Dominican Republic to New York for MRIs revealed no significant concerns, but he again complained of an issue last week after tossing changeups on flat ground, prompting team physician Dr. Christopher Ahmad to organize what was intended to be a three-day battery of tests in New York.

Severino was examined by both Ahmad at New York-Presbyterian Hospital and Altchek at the Hospital for Special Surgery. Cashman said that a dye contrast MRI taken on Tuesday revealed the partial UCL tear. Severino also underwent nerve conductor tests and a CT scan, both of which were negative.

“My immediate reaction is just kind of feeling for him, wanting to check in with him and seeing how he's feeling,” manager Aaron Boone said. “Obviously, it's losing a great pitcher, but we'll manage it. We'll figure it out and just try and support Sevy right now.”

After being limited to just three regular-season starts last year by injuries to his rotator cuff and lat muscle, Severino will likely miss at least part of the 2021 campaign as well.

“My gut is, it's something that dates back to when he started feeling something [during the ALCS],” Cashman said. “The prior MRIs had no problem, and the point of injury was not around the ligament. Our athletic trainers, our physical therapists and the orthopedics who examined him both in Tampa and New York could not produce anything that showed the ligament issue in any shape or form [until Tuesday].”

After seeing a Major League-record 30 players serve 39 stints on the injured list in 2019, the Yankees are experiencing a hangover of sorts this spring. Severino will join left-hander James Paxton (back surgery) on the IL to open the season, while outfielder Aaron Judge’s spring debut has been delayed by soreness in his right shoulder.

• Boone: Judge's timetable coming into focus

“It's crazy how we're getting hit by the injury bug again,” said first baseman Luke Voit. “I feel for Sevy. That sucks, because he [rehabbed] all last year, too. I hope it's a speedy recovery.”

This is the second season of a four-year, $40 million contract extension signed by Severino, whose combination of high-90s heat and plus secondary pitches established his place as the Yankees’ ace in 2017-18. He went 33-14 with a 3.18 ERA and averaged 10.5 strikeouts per nine innings across those two seasons, placing in the top 10 in AL Cy Young Award voting after each campaign.

Catcher Kyle Higashioka caught Severino in the bullpen earlier this spring and spotted no issues,­ though Severino was throwing only fastballs and sliders at the time.

“I'm sure he's just devastated to find the news that he's going to have to spend another season out,” Higashioka said. “I think as a team, we're all just really feeling for him right now.”

The announcement changes the landscape of a rotation that already expected to be without Paxton until at least May. Ace Gerrit Cole projects to be followed by Masahiro Tanaka and J.A. Happ, with Jordan Montgomery seen as a front-runner for one of the back-end slots.

Boone has mentioned right-handers Deivi Garcia, Jonathan Loaisiga, Michael King and Clarke Schmidt among those the club is considering in competition to make the final starting five.

“We're certainly hopeful that we have a lot of hungry, talented personnel wanting to make a name for themselves or continue their journey,” Cashman said. “Losing high-caliber players like Paxton or Sevy is going to provide that lane or that opportunity for someone to step up and take. We'll play this spring out and see what it looks like.”