David Peterson is headed back to the bullpen.

Amid the left-hander’s early season struggles, the Mets are moving Peterson to a relief role with the expectation he will be available to pitch as soon as Sunday.

Peterson could be used in “a lot of different roles,” including for multiple innings or as a traditional left-handed reliever, manager Carlos Mendoza said.

Sean Manaea — who is 0-1 with a 5.56 ERA in 12 appearances, all out of the bullpen — will now get an opportunity as either a starter or as the bulk pitcher behind an opener.

“Obviously, [Peterson] understands the situation,” Mendoza said before Friday night’s series opener against the Miami Marlins at Citi Field.

“It’s what’s best for the team right now, but I also told him that it’s an important role, that we’re gonna need him. This is something that will continue to be fluid. The performance matters here, too, and if he throws the ball well, the way he’s capable of, he’s going to continue to get opportunities.”

Peterson allowed six runs on 11 hits over five innings in his last start, a 7-2 loss to the Cincinnati Reds on Tuesday night.

That dropped Peterson to 3-5 with a 5.57 ERA this season.

The Mets have used Peterson both as a traditional starter and as the bulk pitcher behind an opener this year, and the results have been dramatically different.

In seven starts, Peterson is 1-5 with a 7.56 ERA.

In five relief appearances, all of which exceeded three innings, Peterson is 2-0 with a 2.25 ERA.

But Mendoza acknowledged Friday that Peterson could now be used in high-leverage situations if the right matchups present themselves.

“He’s too good of a pitcher, and we’ve got to figure it out,” Mendoza said. “One thing that David does, he’s not going to shy away from it. He’s gonna keep going. He’s going to continue to work and watch film and try to figure out what’s going on here.”

Peterson, 30, was an All-Star last season, but he struggled down the stretch, pitching to an 8.42 ERA over his final nine starts.

Mendoza said it’s difficult to pinpoint one main culprit for Peterson’s issues this year.

“It’s been a combination of him missing arm-side at times, not executing glove-side, pitch selection at times,” Mendoza said. “It’s a combination of a lot of things, but again, he’s still a very good pitcher, and we’ve seen that.”

SENGA’S START

Kodai Senga (back) feels fine physically after his second rehab start, and he’s set to throw a bullpen session on Sunday before making another rehab appearance.

The right-hander allowed three runs (two earned) with five strikeouts in 3.2 innings for Triple-A Syracuse on Thursday night, surrendering four hits, two walks and a hit-by-pitch over 80 pitches.

“We’re getting to a point where the results are important, too. There were three-ball counts a lot last night,” Mendoza said Friday.

“Right now, it’s more [about] the physical part, but it’s gonna get to a point where results matter.”

Senga, 33, went 0-4 with a 9.00 ERA in five starts before he landed on the injured list with lumbar spine inflammation late last month.

In his two rehab starts, Senga has a 4.91 ERA over seven innings.

ALVAREZ UPDATE

Francisco Alvarez is making quick progress in his recovery from knee surgery and could start playing in rehab games as soon as next week.

The catcher underwent surgery in mid-May for a torn meniscus in his right knee.

“He hit earlier today. He looks pretty normal. It’s amazing,” Mendoza said. “Two weeks from surgery and this guy is doing everything. He’s catching bullpens. He’s blocking, throwing, running.”

Alvarez’s progression will include a build-up to where he’s able to catch nine innings.

“The question will be, ‘Is he gonna catch back-to-back [games for] nine innings in the minor leagues before we put him out here, or is it one of those where we can continue to build him up [in the majors]?’ ” Mendoza said.