Mets’ David Peterson thriving in many different spots as playoff reliever

All pitchers, and starters in particular, rely on routines.

Four days’ rest is normal, and between-outing habits must be adjusted when a day of rest is added or subtracted.

Which makes David Peterson’s postseason all the more impressive.

The Mets lefty made 21 starts this season — generally excellent, with a 2.90 ERA — before taking on a new role as a big-time arm who can be deployed in any significant scenario.

Boxed out by Sean Manaea, Luis Severino, Jose Quintana and a stretching-out Kodai Senga for playoff starts, Peterson instead has logged his first career save, taken down three excellent innings in Game 1 of the NLDS and shut down the Phillies in a potentially swing moment of the deciding Game 4.

“I can see why it would be a big deal. But for him, it’s not,” pitching coach Jeremy Hefner said on the field as the Mets celebrated the clincher Wednesday at Citi Field. “He’s treating it just like he always is. He’s prepared, he’s got a plan, he knows what he’s trying to do, and then it just becomes an execution game.”

Peterson is executing and not complaining, finding a different way to impress in each game he appears.

First Peterson, on three days’ rest, came out of the bullpen in the decisive game against the Brewers, replacing a tired Edwin Diaz and needing just eight pitches to record the final three outs.

Two days later, Peterson watched Senga pitch two solid innings in Philadelphia and entered for the third.

For three scoreless innings, the 29-year-old pitched around traffic (one hit, three walks) and made every big pitch he needed to in a victory that the Mets’ staff stitched together.

The latest challenge for Peterson arrived Wednesday when he entered a game in the middle of an inning for the first time this season.

Inheriting a two-on jam from Quintana and Reed Garrett in the sixth inning, Peterson needed just three pitches to get Bryson Stott to ground out and escape.

“In the playoffs, it’s a challenge whether it’s men on, no one out, starting — whatever it is,” Peterson said after delivering 2 ¹/₃ scoreless innings in the win over the Phillies. “We’re fighting to continue our season.”

Follow The Post’s coverage of the Mets in the postseason:

• Mets overcome arch rival Phillies to advance to NLCS as Lindor hits game-winning grand slam
• Francisco Lindor lights Citi Field on fire with wild go-ahead Mets grand slam
• Brandon Nimmo waited over a decade for this special Mets clincher
• Mets not a better team than us: Phillies’ Nick Castellanos

The Mets’ season continues in large part because a bullpen without a trusted lefty — Danny Young has not pitched an inning in October — has found one capable of getting big outs and a lot of outs. In all, Peterson has 6 ¹/₃ shutout innings in this year’s playoffs, a mid- or late-game weapon whom manager Carlos Mendoza is eager to use.

As the Mets continue to stretch out Senga, who should be ready for three innings in his next outing, Peterson is the best bet to piggyback Senga’s effort.

The pairing was ideal in Philadelphia, and each pitcher complements the other well.

Such a dynamic would not work if Peterson’s mind or body would not allow him to adjust to a different role on the fly as well as he has.

Amid an up-and-down career, an excellent 2022 followed by a rough 2023 and hip surgery before a breakout 2024, Peterson has established himself as not just a solid option but a flexible one.

“I’m so happy for him because it’s been not a super clean career,” Hefner said. “And to have these moments now, in back-to-back series, it’s really awesome.”