The ailing Mets’ rotation is about to take its first step back to being whole again.

Tylor Megill will come off the injured list on Friday to start the series opener against the Angels in Anaheim, his first action since May 11 after being sidelined with biceps inflammation.

Max Scherzer and Jacob deGrom remain on the injured list, at least for a few more weeks, but Megill’s return could give the Mets a boost if he can pitch like he did over the first month of the season as deGrom’s replacement.

“It’s important for everybody, but especially Tylor, [who] was pitching real well for us for the most part when he went down,” manager Buck Showalter said Wednesday night before the Mets left San Diego. “So was Max. And we all know what Jake’s capable of.

“That’s the way baseball works. When you play this long, you plug one hole — I don’t want to call it a hole because we got some pieces that are very capable. You get one thing back and something else goes away. I don’t think San Diego — for instance, without [Fernando] Tatis — really cares about our problems. So, hang with ’em.”

Since Megill and Scherzer both landed on the IL in a span of five days in May, the Mets have gone 13-7 while increasing their lead atop the NL East from six games to seven (before the Braves played Thursday night).

Their rotation during that span has pitched to a 5.28 ERA, which ranks 23rd in the majors — after its ERA of 3.23 ranked fifth through May 18, the day before Scherzer joined Megill on the IL.

The main culprit has been Chris Bassitt, who owns a 7.62 ERA (22 earned runs in 26 innings) over his past five starts after his latest rough outing Wednesday night against the Padres. The Mets’ inflated ERA also includes Thomas Szapucki’s spot start in San Francisco, when he got shelled for nine runs in 1 ¹/₃ innings.

Excluding Bassitt and Szapucki, the Mets’ rotation of Carlos Carrasco, Taijuan Walker, and fill-ins Trevor Williams and David Peterson have registered an ERA of 3.48 since May 19.

Megill, who made a rehab start on Sunday for Double-A Binghamton, is expected to be stretched out enough to throw 70-75 pitches on Friday. Williams will be in the bullpen to provide length if needed.

Across his first six starts of the season, Megill posted a 2.43 ERA with 36 strikeouts in 33 ¹/₃ innings. But in the right-hander’s most recent start, the Nationals tagged Megill for eight runs in 1 ¹/₃ innings, which may have been impacted by his budding biceps issue.

Now, Megill’s return will come in Anaheim — about 30 minutes away from his hometown of Long Beach, Calif. — against an Angels team that fired manager Joe Maddon this week and brought a 14-game losing streak into Thursday night.

Carrasco and Walker will follow on Saturday and Sunday, with the Mets using Thursday’s off day to skip Peterson. They will not need a fifth starter again until June 18.

“We’ve got some idea of where he’s going to slot back in and start again,” Showalter said. “But we’ll see how the next few days play out.”

The earliest deGrom could return from a stress reaction on his right scapula is by the end of June. He threw his first bullpen (19 pitches, all fastballs with moderate intensity) on Saturday since being shut down in spring training. His next step would be facing live hitters before going out on a rehab assignment as he continues to build up.

Scherzer, meanwhile, was given a six to eight week timeline for the strained oblique he sustained on May 18. That would put him on track to return sometime around early July, possibly around the same time as deGrom.