PHILADELPHIA — Kodai Senga’s quest back to the majors and back to Mets relevance begins Friday.

The one-time ace will make his first start with Triple-A Syracuse after accepting a minor league option last Friday amid significant struggles at the major league level.

Senga is expected to make two starts to demonstrate to himself and the team that he has straightened himself out in time for what could be one final regular-season major league start.

Since being demoted, Senga has been throwing bullpen sessions and working on his mechanics, which he partially blamed for a miserable, eight-start stretch in which he pitched to a 6.56 ERA.

Kodai Senga reacts during his Aug. 31 start against the Marlins. Robert Sabo for the NY Post

Kodai Senga prepares to throw a pitch during the Mets’ Aug. 31 game against the Marlins. Robert Sabo for the NY Post

“He feels good. He feels like he’s in a good spot,” manager Carlos Mendoza said of Senga after those targeted bullpen sessions. “He’s made some adjustments with his mechanics. Now he’s got to go out on the mound and we got to see.”

The Mets have six starting pitchers at the moment and could get “creative,” as Mendoza has said, with the rotation after Monday’s off day.

They could ask starters such as Sean Manaea and Clay Holmes to pitch in shorter bursts — say, three or four innings while piggybacking off of another pitcher — because pitching deeper into games has been a problem while their stuff itself often has not.

Senga could figure in to the equation in about a week and a half: If he starts Friday and then the following Thursday and performs, maybe he would be activated for a Sept. 24 return.

He is not eligible to be brought back, barring an injury to another Mets player, until Sept. 20.

But the Mets likely will have to see Senga pitching like he did before straining his hamstring in June — before which he owned a 1.47 ERA — to feel comfortable adding him to the mix.

“Now he’s facing competition [Friday],” Mendoza said before a 6-4 loss at Citizens Bank Park. “A lot of times the hitters will tell you [how his stuff plays].”

David Peterson, who entered sporting a 7.42 ERA in his past six starts, was better but not the All-Star he became in the first half.

The lefty’s stuff was sharp and resulted in eight strikeouts, but he lasted just five innings in which he let up three runs because of a few mistakes.

“Felt good mechanically,” Peterson said after battling through three scoreless innings before the troubles arose in the fourth. “There’s a couple pitches there that they ended up taking advantage of.”

David Peterson reacts during the Mets’ loss to the Phillies on Sept. 11. Corey Sipkin for the NY Post

In that fourth, he could not put away Nick Castellanos on an eight-pitch walk, then threw a first-pitch, down-the-middle slider that Otto Kemp punished 433 feet to center to cut the Mets’ 4-0 lead in half.

In his next and last inning, Peterson allowed a leadoff single to Weston Wilson before retiring Harrison Bader and Kyle Schwarber.

One out from an escape, Peterson watched Harper place a double into the right field corner to make it a 4-3 game.

“Good one to build off of,” Peterson said.

Wearing an “NYPD” hat on Sept. 11, Mendoza began his pregame news conference with a statement:

“As I get to put on this uniform, this hat — what it represents on a day like today. Honoring, remembering all the people and the families that were lost on September 11,” Mendoza said. “Continuing to pray for those families.”

The Mets did not have further information regarding Tylor Megill, who is seeing multiple doctors after a recurrence of elbow tightness while on his rehab assignment.

Luis Torrens, who was placed on the IL with a right forearm contusion Monday, is feeling “better,” Mendoza said.

The swelling has subsided, and the Mets are hopeful he can miss the minimum 10 days.

Tyrone Taylor (hamstring strain) had been expected to begin a rehab assignment next week but could be delayed because he is going home to his wife, who is expecting a baby any day now.

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