The team said Tuesday the 26-year-old right-hander has a partially torn ulnar collateral ligament, a development that means the two-time All-Star will miss all of 2020 after being sidelined for nearly all of 2019.
Mets medical director Dr. David Altchek will operate Thursday at the Hospital For Special Surgery in New York.
Severino released a statement on Twitter after the decision was made to have surgery Tuesday.
Severino was scratched from throwing his second bullpen session in camp last week and didn't take part in fielding drills for pitchers. After the workout, manager Aaron Boone said Severino had been dealing with forearm discomfort that started after Game 3 of the American League Championship Series against the Houston Astros.
The Yankees will already be without pitcher James Paxton through at least April after the left-hander underwent a procedure to remove a cyst as well as a microscopic lumbar surgery. Domingo German must serve the final 63 games of an 81-game suspension for violating Major League Baseball's domestic violence policy. He is eligible to return June 5, barring any postponements.
Severino's injury leaves the AL East favorites with a rotation of newly signed ace Gerrit Cole, Masahiro Tanaka, J.A. Happ and two openings. The usual recovery time for Tommy John surgery is a year or more.
"I don't want to sugarcoat the fact that being without Sevy, that's a blow, but it doesn't change our expectations and what we're truly capable of," Boone said. "So, no, nothing changes."
Lefty Jordan Montgomery, who came back in September from Tommy John surgery, has been throwing at up to 94 mph and is the leading candidate for one slot. Jonathan Loaisiga and rookies Deivi Garcia and Michael King are also possibilities along with Luis Cessa.
"We're always constantly looking for upgrades anyway," Yankees general manager Brian Cashman said. "But this time of year you always look from within and see and give opportunities for what you have and typically that's how it shakes out, especially until after the June draft regardless, so you keep relying on depth. So wouldn't say expect any domino effect or cause and effect in terms of us being able to go to marketplace where a marketplace this time of year typically doesn't exist."
New York won the AL East with a 103-59 record last year, its best in a decade, and lost to Houston in the AL Championship Series.
Cashman said last week that Severino had two MRIs -- one in December and another in January -- and a CT scan after complaining of discomfort, and all tests had been negative. But the Yankees sent him for further testing. Cashman said Tuesday that a dye contrast MRI revealed the partial tear of the UCL.
In January, Severino was treated with anti-inflammatories and said he felt fine, Boone said. Testing revealed a "loose body" near his elbow, which the team believes is an incidental unrelated finding. Once Severino ramped up his throwing program this spring, he stayed away from his changeup.
Severino, who turned 26 on Thursday, didn't pitch in the majors last season until September after spending most of the season on the IL because of injuries to his right rotator cuff and lat muscle. He made three starts.
The two-time All-Star was coming off his best season in 2018 when he went 19-8 with a 3.39 ERA and 220 strikeouts in 191⅓ innings.
After that success, Severino signed a four-year, $40 million extension almost exactly a year ago, but he has thrown only 20⅓ innings since then.
ESPN's Marly Rivera and The Associated Press contributed to this report.