PHILADELPHIA -- The good news, for the Mets, is that their pitching replacements have been something close to perfect. Less than a week after Tylor Megill earned accolades with five scoreless innings on Opening Day, David Peterson delivered four scoreless relief innings in Monday’s 5-4 loss to the Phillies.

The bad news, for the Mets, is that the season is not yet a week old, and they have already needed multiple such replacements. Peterson’s effort was necessary because Taijuan Walker lasted only two innings at Citizens Bank Park before departing due to right shoulder irritation. Later in the game, reliever Trevor May departed because of discomfort in the back of his right arm. Both he and Walker will go for MRIs on Tuesday to determine the exact nature and severity of what’s ailing them.

And of course, the Mets are already playing without their ace, Jacob deGrom, whose right shoulder injury is likely to keep him out until at least June.

“This one hurts,” manager Buck Showalter said.

Showalter was referring mostly to Monday’s endgame, which saw the Mets squander Peterson’s fine effort when three other relievers combined to allow five runs in the eighth. After May departed alongside a trainer, Joely Rodríguez allowed a two-run homer to J.T. Realmuto. Seth Lugo then walked Nick Castellanos with two outs and gave up consecutive doubles to Rhys Hoskins and Didi Gregorius -- the first of them tying the game, the second putting the Phillies ahead for the first time all night.

But Showalter may as well have been referring to the state of his pitching staff. Although everyone involved -- Showalter, Walker, May -- downplayed the severity of the two injuries, the Mets won’t know anything for certain until they receive the MRI results.

“You want to rule things out,” May said. “There’s no reason to wait for certain things, especially when you can know tomorrow. I’ve had MRIs for all kinds of stuff in the past, and this is pretty much protocol all the time.”

May described his issue as a mix of biceps and triceps soreness that typically affects him in Spring Training. He indicated that coming out for a second inning of work, which he did not do a single time in 2021, may have affected him, as he was unable to loosen his arm in the eighth.

Walker described his own injury as a mild bout of soreness and irritation that was “already feeling better” after the game, as corroborated by a series of strength tests.

“I feel pretty good about it,” Walker said.

Had the Mets managed to hold onto their four-run lead, they likely would have departed Citizens Bank Park without much concern at all. As it was, they could still revel in a few positive vibes surrounding Peterson, who struck out three batters and allowed five baserunners over his four scoreless innings. After not making the Opening Day roster, Peterson traveled with the Mets on their taxi squad, then was elevated to the active roster when Edwin Díaz went on the bereavement list over the weekend.

All the while, Peterson stayed sharp, understanding the fluid nature of pitching staffs. Because Walker had exited his final Spring Training start due to a right knee injury and was not completely stretched out, Peterson arrived at the ballpark on Monday figuring he would be used at some point.

“If you let not making the team out of Spring Training get to you,” Peterson said, “then you’re not going to be ready when it’s your time.”

Now, he’ll stay ready; if Walker must miss a rotation turn or more, Peterson would be the obvious candidate to replace him. Regardless of what happens with May, the Mets will also receive a bullpen boost on Tuesday, with Díaz scheduled to return from bereavement. (The team will either need to option a player to the Minors or place someone on the injured list to clear space.)

Whether these are indeed short-term injuries or something more sinister, the Mets are prepared to deal with them. They’ve already done so with deGrom. They’ll do so with various others over the course of a long season.

“We’ve always put an emphasis on depth, especially pitching-wise,” Peterson said. “My job is to go out there and get outs. In whatever capacity that’s at, in whatever spot I’m needed, I’m there.”