MILWAUKEE — In a season in which the Mets have lost countless lefty options, they found one when they needed one most.
Brooks Raley is hurt.
Jake Diekman is gone.
Alex Young has been optioned, and Danny Young has occasionally lost his control and sunk in the bullpen hierarchy.
In an instant classic on Thursday night, Carlos Mendoza turned to David Peterson for the final three outs of a surreal and heart-stopping 4-2 win over the Brewers at American Family Field that sent the Mets to Philadelphia for the NLDS.
Peterson had most recently pitched against these same Brewers on Sunday, when he helped the Mets win the 160th game of their season that ensured they would only need to win one game of Monday’s doubleheader in Atlanta.
The lefty knew he should be ready for the final innings and watched as Jose Butto gave up the go-ahead runs in the seventh and Edwin Diaz needed 39 pitches to record five outs in the seventh and eighth.
Pete Alonso drilled a three-run home run against star closer Devin Williams in the top of the ninth that guaranteed there would be a bottom of the ninth, when a lefty pitcher would be the best Mets option.
“Everybody went nuts,” Peterson said, “and then [bullpen coach Jose Rosado] and I kind of looked at each other and we were like, ‘OK, we need to reel it back in.’
“Just go out there and do what I needed to do.”
What Peterson, who has pitched 16 times in his five major league seasons out of the bullpen, needed to do was record his first career save.
Given a two-run lead, Peterson watched Sal Frelick open the frame with a single.
“I tried to slow down and keep my heart rate down as much as possible,” said the southpaw, who was the pick in part because the Brewers were sending two lefties to the plate. “I knew that the crowd and the energy of the game and the situation was going to provide everything I need.”
He used a four-seam fastball to strike out Joey Ortiz before Brice Turang — whom the Mets could not retire all series — bounced a grounder to Francisco Lindor, who rushed to second and threw to first to finish it off.
“Helluva job by Diaz and then Peterson getting the last three outs,” Mendoza said.