As the trade deadline approaches, the Mets will likely make one addition that won’t cost them any players: the activation of Trevor May.

The hard-throwing right-hander is set to begin a rehab assignment with Double-A Binghamton on Sunday, Mets manager Buck Showalter said. May will be pitching in a game for the first time since May 2.

“Been a long road for him, but he’s been very diligent,” the manager said Saturday before the Mets hosted the Padres. “We’ve really missed him.”

Early in the season, May had issues with his triceps. He tried to pitch through it, but could not. Imaging found a stress reaction in the lower part of his right humerus, and he was shelved after pitching just 8 ¹/₃ innings this year.

The 32-year-old was a big part of the Mets’ bullpen last season, when he pitched to a 3.59 ERA with 83 strikeouts in 62 ²/₃ innings.

The Mets need relief reinforcements, especially after trading Colin Holderman in the Daniel Vogelbach swap and they are expected to be heavily involved in the bullpen trade market. May will need a few minor league outings and has said his goal is to be ready by early August.

“I didn’t really get to see that Trevor May [from last season],” Showalter said. “I’m looking forward to getting him back and putting him in the mix out there.”

Jacob deGrom is set to throw Sunday at Citi Field before he makes what could be his final rehab start.

Showalter said the Mets likely will have to wait to see how the ace, whose simulated game last week was pushed back two days because of “mild muscle soreness around his shoulder,” feels Monday before deciding the specifics of the rehab start.

Fresh off the four-day All-Star break, the Mets will be idle Monday and Thursday, a scheduling quirk Showalter is not thrilled about.

“Doesn’t make any sense,” Showalter said. “Figure that out with the schedule. I’d love to hear that explanation. … I’d love to take these two days and move them to August, when you really need them.”

Showalter suggested he would try to mix in each reliever in the coming days amid the long layoff.

The Mets are expecting large crowds for the Subway Series games Tuesday and Wednesday at Citi Field. They are encouraging fans to take mass transit and arrive early. For the two games, the Mets will offer $5 hot dogs at concession stands from 5:10-6:10 p.m.