Kodai Senga took a step back into the bullpen and took a step forward in performance.

In his first major league outing as a reliever, Senga was much sharper than he had looked as a failed starter, even if the home run he served up to Kyle Schwarber became the difference in a 5-4 loss to the Phillies at Citi Field on Sunday.

“He was really good today,” interim manager Andy Green said after Senga allowed two runs on four hits and one walk while striking out four in five innings.

The 2023 All-Star and 2024-26 mystery had fallen out of the rotation after seven games in which he owned a 10.08 ERA.

But Senga — who was needed on an afternoon the Mets essentially threw a bullpen game while four of their relievers were not available because of recent workload — entered in the fifth without complaint, saying the Mets afforded him plenty of time to warm up for a job that “I haven’t done” often.

He had no issues with Trea Turner, Schwarber and Bryce Harper and looked as if he had rediscovered his forkball.

He encountered trouble in the sixth, walking Alec Bohm before a Brandon Marsh double, but Senga used forkballs to strike out J.T. Realmuto and Derek Hill, sandwiched between a Bryson Stott groundout.

Senga remained for the seventh, when he faced the top of the Phillies order for a second time because Green had no other reliever to turn to.

Senga got ahead of Schwarber 1-2 before missing down the middle with a fastball that was demolished for a game-swinging two-run homer.

Senga acknowledged that he could have chosen a different pitch or at least hit his spot better, trying to throw the fastball up and out of the zone.

“You get to 1-2 on one of the best hitters on the planet, you want to locate better,” Green said. “I don’t want to take anything away from his outing — he came out of the bullpen, he gave five really strong innings.”

The Mets, who are using four starting pitchers and have an off-day Thursday that delays the need for a fifth, very much could use a reemergence from Senga, but that hope has dwindled.

Senga said his role is ultimately “up to me” because his performance will dictate how he is used.

“If I can’t compete and be in the zone … then I don’t deserve a spot in the rotation,” Senga said through interpreter Hiro Fujiwara. “But on the flip side, if I am able to compete, I do feel like I can throw in the rotation.”