Red Sox pitcher Chris Sale works against the Tampa Bay Rays at Tropicana Field in a game last July.

Red Sox ace Chris Sale will undergo Tommy John surgery to repair the ulnar collateral ligament in his pitching elbow, the team announced Thursday. Sale will miss the entire 2020 season, which has been delayed due to the ongoing coronavirus pandemic, and potentially some 2021, as well.

Sale, who missed the final six weeks of last season with elbow inflammation, entered spring training with no arm pain but was delayed in his progression due to a battle with pneumonia in mid-February.

After facing hitters for the first time in six months during a March 1 live batting practice session, Sale felt a recurrence of pain in his elbow and underwent an MRI that determined he had a flexor strain in the elbow. Doctors prescribed 10-14 days of rest and Sale himself admitted that any further pain would likely require a surgical option.

Sale reportedly restarted his throwing program in recent days, so Thursday’s announcement suggests he still felt discomfort after picking things back up. It’s unclear exactly when he will undergo surgery but the expectation is that it will take place in the coming days.

Sale, who signed a five-year, $145 million extension a year ago, struggled to a 6-11 record and 4.40 ERA in 25 starts last season before hitting the injured list with inflammation on Aug. 17. At that point, he received a platelet-rich plasma injection and was shut down for six weeks, with the team further delaying his rehab once it was clear the Red Sox would miss the postseason.

The Sox entered 2020 with confidence about the state of Sale’s elbow, and, in the immediate wake of the March 1 session, believed he would be ready to start games about two weeks into the season. The setback further delayed that timeline, with the team looking at a one-month absence as the best-case scenario after the flexor strain diagnosis.

Because baseball, much like every other sport, is currently in limbo due to the coronavirus pandemic, it’s impossible to know just how much time Sale will miss. Tommy John surgery usually requires a 12- to 15-month recovery period, so even if a significant portion of the 2020 season is played, Sale will not participate.

A best-case scenario for him, assuming he undergoes the procedure in the coming days, would have him returning sometime in summer 2021.

If baseball is played in 2020, the Red Sox’ rotation will look much different than it did in 2019. With Sale hurt and both David Price and Rick Porcello now in the National League, Boston will proceed with three set starters — Eduardo Rodriguez, Nathan Eovaldi and Martin Perez — and a collection of unheralded options like Ryan Weber, Collin McHugh and Brian Johnson.

The Sox appear likely to use an opener frequently once the season begins and could be forced to get even more creative once its clear how Major League Baseball plans to alter its schedule to accommodate the coronavirus-caused delay.