ST. LOUIS — Jesse Winker started his first game of the season in left field on Sunday and by the afternoon’s conclusion was receiving an MRI exam.

The experienced outfielder left Game 1 of a doubleheader against the Cardinals due to discomfort in his right side after trying to throw a runner out at home.

Winker received an MRI exam on his oblique and was planning Sunday night for a return to New York for further testing.

Following the Mets’ losses in the doubleheader — with scores of 6-5 and 5-4 — manager Carlos Mendoza indicated that Winker is likely to be placed on the injured list ahead of Monday’s game in Arizona.

Potential candidates to fill in as a left-handed hitter might include Brett Baty. However, Mendoza noted that Baty has been out of action since Thursday at Triple-A Syracuse because of a sore big right toe.

Even so, Mendoza said Baty has been working out pregame in recent days.

For the nightcap, Brandon Nimmo returned to left field — he got a partial rest as the DH in the first game — and Starling Marte started in right field.

Juan Soto was used as the DH.

Marte was removed after four innings, as was Mendoza’s plan because the veteran was playing in the outfield for the first time this season.

Mendoza was asked if rust may have affected Winker (the team’s primary DH) from the physical standpoint of playing the outfield for the first time this season.

“He’s been doing everything he can to stay ready, doing all the defensive work pregame,” Mendoza said.

Winker caught Nolan Gorman’s fly ball to medium left field in the third inning and Brendan Donovan beat the throw home.

The inning ended with Nolan Arenado caught in a rundown between third and home after Luis Torrens threw to second base.

Winker was removed from the game before the Cardinals batted in the fourth.

Dedniel Núñez was recalled from Triple-A Syracuse before the second game to give the Mets an eighth reliever.

Sixth starter Blade Tidwell was optioned back to Syracuse.

The right-hander Núñez was a key piece of the Mets bullpen last season before he was shut down with forearm tightness.

He spent the past month in the minors ramping up and waiting for an opportunity with the Mets.

“When I got sent down it gave me the opportunity to work on my pitches,” Núñez said through an interpreter. “It also gave me time to get into game shape.”

Surgery has been recommended for reliever Danny Young, who was placed on the injured list last week with an elbow sprain.

Young was transferred to the 60-day IL on Sunday, giving the Mets an additional spot on the 40-man roster.