Mets starting pitcher David Peterson walks to the dugout after...

Mets starting pitcher David Peterson walks to the dugout after being taken out of the game during the first inning after the Cubs scored four runs in an MLB game at Citi Field on Wednesday. Credit: Kathleen Malone-Van Dyke

Up next for David Peterson is a step into a great unknown: the bullpen.

The Mets will transition Peterson into a relief role — with which he has dabbled with in months and years past but never had to do for an extended stretch — potentially as soon as Friday, manager Buck Showalter said. This amounts to a late-season experiment to see if he can serve as the sort of effective lefty out of the ‘pen that the Mets have lacked most of the season, but it comes with questions.

Will Peterson be good at this job? Will he be better at short or long relief? How will his arm bounce back with an unusual pitching schedule?

“How is Pete going to respond?” Showalter said. “If he throws 18 pitches to three hitters, is he going to be able to scratch his rear the next day? I don’t know.”

Showalter noted that the Mets “like him as a starting pitcher down the road.” But with Max Scherzer (left side irritation) due back Monday, the rotation will become whole again, squeezing out Peterson, whose one-out, five-run mess of a start Wednesday belied his strong season overall (3.91 ERA).

So if Peterson is going to help the Mets in the final weeks of the regular season — and then the playoffs — they think it will come as a reliever.

Alex Claudio “has done a good job,” Showalter said, but that is based on just three appearances since his callup last week. Joely Rodriguez (5.18 ERA) “has had his moments too,” the manager added.

Hence, curiosity about Peterson.

“Just because he throws 90-some-odd miles an hour doesn’t mean he’s going to be able to do well in that role,” Showalter said. “I don’t know for sure.”

Extra bases

Hitting coach Eric Chavez left the team until at least Monday to tend to a family matter, Showalter said . . . Francisco Lindor was going to be the DH on Thursday until Showalter remembered it was Roberto Clemente Day and figured Lindor, a Puerto Rico native, probably preferred to play the field. He hit a two-run homer in the third inning to make it 4-1 . . . Showalter on Scherzer, a day after his rehab start: “The body language is really good.” . . . Tomas Nido was “pretty sore” after getting hit by two balls Wednesday, according to Showalter: one off his knee and one “in a real uncomfortable place that he’s very sore in today. Everybody figure out what I’m talking about?”

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