BALTIMORE — Justin Hagenman is new enough to the MLB scene that each pitching appearance for the Mets can be considered a learning experience.

He had pitched multiple innings in games against the Twins and Braves, but last week he was unexpectedly chosen as the Mets’ starting pitcher for the Subway Series.

The right-hander got jumped early, but ultimately helped keep the Mets from folding in a comeback victory.

During the game, Hagenman gave up back-to-back home runs to Jasson Domínguez and Aaron Judge in the first inning. Despite this rough start, he managed to hold off the Mets until the fifth inning, allowing four earned runs in total.

Reflecting on his performance, Hagenman commented, “Regardless of how the game begins, I believe in my ability to compete. While it wasn’t the ideal start, I was able to regain my composure and continue pitching effectively against the lineup.”

The Mets have to survive a final day before the rotation reinforcements begin arriving.

It’s the 28-year-old Hagenman they are expected to look toward in Thursday’s doubleheader nightcap — either as a starter or behind an opener — provided he isn’t needed from the bullpen in the opening game.

Kodai Senga is scheduled to return from the injured list on Friday to start in Kansas City and Sean Manaea’s season debut is set for Sunday as part of a piggyback with Clay Holmes.

Hagenman, who arrived on a major league contract last offseason for the purpose of providing rotation depth, has pitched to a 4.35 ERA in his three appearances.

“When I signed I was just hoping to get a shot, to get an opportunity to help the team up here and I have,” Hagenman said. “It’s been nice just to be an option for the guys up here, you never know when it might be.”

Hagenman spent two weeks on the injured list in early May at Triple-A Syracuse with a finger injury and returned with better velocity and command of his pitches than the Mets saw earlier in the season, according to pitching coach Jeremy Hefner.

“He got a little two-week blow and ever since then he’s come back, he is throwing harder,” Hefner said. “It feels like the slider is firmer and that is helping. The cutter is in a good spot and the changeup has always been his calling card and that is showing very well. He is a high-command guy. He is going to put the ball in the zone, he is going to live on the corners and he’s going to be able to kind of navigate games.”

Hagenman’s resolve to keep the Mets in last Friday’s game against the Yankees impressed Hefner.

“That speaks to his demeanor,” Hefner said. “He can be feeling things inside, but it doesn’t manifest externally.”

Hagenman grew up near Philadelphia, in Voorhees, N.J., and attended a game at Camden Yards in his youth.

In that sense he will be living another dream should he get to pitch. Hagenman said he will have several friends and family members in attendance.

Last month he was recalled to the Mets for a potential start in Philadelphia, but he was needed in a relief appearance against the Braves a night earlier, removing him from consideration for the assignment.

“There were going to be a lot of ticket requests on that one,” Hagenman said. “But pitching in Atlanta was cool, against that lineup which is pretty awesome too. It was a win-win. Any time you get to pitch up here it’s a win.”