Mets pitcher Seth Lugo delivers during the first inning of...

Mets pitcher Seth Lugo delivers during the first inning of a game against the Nationals in Washington on Sept. 27, 2020. Credit: AP/Nick Wass

Days before spring training, the Mets already have suffered a significant injury, losing their most reliable reliever to elbow surgery.

They announced Saturday that righthander Seth Lugo will go under the knife Tuesday to remove a "loose body" in his right elbow. He won’t throw for at least six weeks, meaning he will miss all of spring training — and thus a big chunk of the regular season.

"While going through his normal throwing progression during the offseason, a bone spur had broken off at some point," the Mets said in a statement. "Inflammation that wouldn’t subside recently caused Seth to inform the Mets' medical staff of the situation and they ordered an MRI."

That MRI happened Friday and revealed the broken-off bone spur. Dr. David Altchek, the Mets’ medical director, will perform the surgery at the Hospital for Special Surgery in West Palm Beach, Florida.

If Lugo has a normal spring training-style ramp-up after his rest period, that would mean he is out until at least mid-May.

That is a huge blow to a bullpen that already is a potential weak spot for the 2021 Mets. Last year, their relievers combined for a 4.60 ERA, including a 2.61 ERA from Lugo before he was moved to the rotation because the Mets ran out of starters.

Lugo has a career 2.53 ERA and 0.95 WHIP out of the bullpen, with most of that work coming in the past three seasons, when he has been the best reliever on the team and one of the best in baseball.

This offseason, the Mets added righthander Trevor May and lefthander Aaron Loup to the bullpen. They also have closer Edwin Diaz, Miguel Castro and several pitchers who struggled last season, including Jeurys Familia, Dellin Betances and Robert Gsellman.