NEW YORK -- The Mets’ rebuilt bullpen took a significant hit on Saturday, when the team announced that Seth Lugo will undergo surgery to remove a loose body from his right elbow. The Mets intend to shut Lugo down from throwing for six weeks, which will take him out of consideration for the Opening Day roster.

In a telephone interview, Lugo said he felt nothing abnormal in his arm until last week, when his elbow began swelling and it would not subside. After a few days, Lugo called someone from the Mets’ training staff, who scheduled an MRI. That test, taken Friday, revealed a bone spur that had broken off his right elbow.

Lugo will undergo surgery on Tuesday, but the right-hander -- who has been pitching with a partially torn UCL in his elbow for at least the past four seasons -- called his fate far better than the worst-case scenario.

“I’ve been optimistic since [Friday],” Lugo said. “Last week, I was sitting there thinking, ‘If I tore [a ligament], my career might be over, so this is devastating.’ But I feel really good now that it’s just [the bone spur]. … The worst-case scenario was, I don’t pitch again, ever, so I’m pretty happy.”

It is nonetheless a difficult blow for the Mets, considering Lugo has been their clear best reliever for most of the past three seasons, posting a 2.68 ERA with 207 strikeouts over 181 1/3 innings from 2018-19. Last summer, the Mets attempted to stretch Lugo out as a starting pitcher on the fly, with mixed results; he allowed two runs over his first 11 2/3 innings, before finishing the season with a 9.82 ERA in four starts.

After the season, Lugo began throwing earlier than normal in October because of how strong he felt following the shortened 60-game schedule. It was not until February that he felt any discomfort, which stemmed entirely from the bone spur -- not his UCL.

“I was actually pretty happy about [the diagnosis],” said Lugo, who opted to rehab his UCL rather than undergo Tommy John surgery in 2017. “It was good news for me, minus missing a little bit of time. … This is kind of the first thing that’s ever popped up ever since -- I don’t want to say the UCL. I don’t want to bring it up. With the MRI [on Friday], it looked great. The doctor was really impressed with how it wound up.”

The bone spur is of more immediate concern. A six-week shutdown will take Lugo to the final week of March, mere days before Opening Day. Although Lugo declined to comment on his timeline, he remained upbeat about his overall prognosis. If he is able to begin throwing on schedule, Lugo would likely require another month or more to work into game shape, making May a realistic best-case scenario for his return.

“I’m just going to have the surgery and go from there,” Lugo said.

In the interim, the Mets will have to find somebody else to fill Lugo’s unique role at the back of their bullpen. He had become a true “fireman” for the Mets in recent seasons, often pitching in the highest-leverage spots from the middle innings through the ninth. In that fashion, Lugo was a candidate to continue taking save opportunities away from Edwin Díaz, while also remaining active in the middle to late innings.

The Mets have added two pitchers, Trevor May and Aaron Loup, on big league deals this winter, as well as a swingman, Sam McWilliams, who could conceivably fill some part of Lugo’s role. Díaz, May and Loup are locks for the Opening Day bullpen, as are Jeurys Familia and Dellin Betances -- provided the Mets don’t trade either of those two in salary-dump-type deals. Miguel Castro and Drew Smith also figure to play prominent roles, but Lugo’s injury increases the chance that the Mets continue looking outside the organization for relief help. The team has been linked to free agents Trevor Rosenthal and Justin Wilson, among others, in recent days.

During the early part of this offseason, Mets officials debated whether Lugo would be a better fit in their rotation or bullpen -- a debate that essentially ended when they added multiple pieces to their starting mix. Now, the Mets are simply hopeful that Lugo can be a significant contributor over the final two-thirds of 2021.

“It’s a professional group out there,” Lugo said. “With the guys coming back from last year, I have no doubt that they’ll be able to hold down the games. I’m still looking forward to meeting the new guys that we signed, and hopefully when I come back, we’ll be in good position in the standings. Better now than in August, right?”