Boston Red Sox left-hander Chris Sale underwent what the team called "successful" Tommy John surgery on his throwing arm Monday, a procedure that is expected to sideline him into the 2021 season.

Famed surgeon Neal ElAttrache performed the ulnar collateral ligament reconstruction at the Kerlan-Jobe Institute in Los Angeles, according to a release by the club.

Sale, who turned 31 on Monday, underwent an MRI exam earlier this month that led to the diagnosis of a flexor tendon strain in his left forearm. At that point, ElAttrache and Dr. James Andrews were hoping rest and rehabilitation would allow Sale to avoid surgery.

"I talked to many doctors," Sale said on March 5. "World-renowned. They seemed confident. Everyone agreed -- take some time off, get some anti-inflammatories in there, start another throwing program and see what we get."

Sale, a seven-time All-Star, began throwing again two weeks ago at the team's facility in Fort Myers, Fla., and apparently the elbow didn't respond favorably.

Sale went on the disabled last August due to elbow inflammation and missed the rest of the season.

Sale posted a 6-11 record with a 4.40 ERA in 2019, the worst year statistically of his career since he entered the Chicago White Sox's rotation in 2012. Excluding his time as a reliever in 2010 and '11, he posted career lows in starts (25), wins and innings pitched (147 1/3) and the highest ERA in 2019.

He has a 109-73 career record (35-23 with Boston) with a 3.03 ERA. Sale has 2,007 strikeouts in 1,629 2/3 innings over 312 games (232 starts). His strikeout-to-walk ratio, 5.37, is the best in baseball history.

Sale signed a five-year, $145 million contract extension in March 2019. The deal runs through the 2024 season and will pay him $30 million in 2020.