NEW YORK -- When Mark Vientos arrived at Citi Field on Saturday for the first time this season, Carlos Mendoza approached him for a chat. The manager told Vientos to be ready for every opportunity. He urged him not to worry about the awkwardness of the circumstances, the fact that his stay might only be temporary.

“I’m going to ask you to get probably the biggest at-bat in a game,” Mendoza told Vientos.

Neither man could have known quite how prescient that advice would be. Barely 24 hours later, Vientos approached the plate Sunday in the 11th inning of a tie game. With the Mets one strike away from being swept by the last-place Cardinals, Harrison Bader had just thrown them a lifeline in the form of a game-tying RBI single. Next up was Vientos.

He swung through a fastball at the top of the zone, then took a ball and fouled off two pitches from Matthew Liberatore, a left-hander in his fourth inning of relief. When Liberatore went looking for strike three in almost the same spot as strike one, Vientos jolted it to right-center field.

As it landed over the fence, Vientos looked back at the dugout, screamed, then pumped both fists. Upon rounding third base, he ripped off his helmet, threw it skyward, and jumped into a mosh pit of his teammates waiting at home. He shouted one more time as the group enveloped him.

That was how the Mets completed a 4-2, 11-inning, walk-off win over the Cardinals at Citi Field, relying on a player with a self-described chip on his shoulder to land the final punch.

“To be honest with you, I feel like it’s almost a déjà vu moment,” Vientos said. “I feel like I’ve lived that moment over and over in my head. It was just like, let it go, let all the energy out.”

For most of Spring Training, Vientos figured he would be part of things when the Mets flew north for another regular season. It wasn’t until the final week of camp that Mendoza and president of baseball operations David Stearns told Vientos he would be heading to Triple-A Syracuse instead. They cited the Mets’ early-season pitching matchups, as well as their desire for the 24-year-old to improve defensively. They acknowledged, as Mendoza put it, that Vientos “wasn’t happy.”

But that was the decision. All Vientos could do was prove what he already had time and again: that he was too good for Triple-A. A .923 OPS in 23 games seemed like evidence enough for Stearns and Mendoza, so when Starling Marte went on the bereavement list Saturday, they brought Vientos back to the Majors.

In doing so, they guaranteed him nothing. With J.D. Martinez on the roster, Brett Baty enjoying some early-season success and Marte potentially back as soon as Tuesday, Vientos doesn’t have a long runway to prove that he deserves to stay. His only recourse, as Mendoza told him in their pregame meeting, was to take advantage of every opportunity.

“He’s happy, but you can tell he’s got a purpose,” shortstop Francisco Lindor said.

Sunday offered the chance for Vientos to prove it. With a larger-than-normal crowd in attendance on a mild afternoon, starting pitchers José Quintana and Lance Lynn pitched well enough to ensure things would stay tight into the later innings. Michael Siani plated a run with a squeeze bunt for the Cardinals in the fifth. Lindor hit a solo homer for the Mets in the sixth. That was all until the top of the 11th, when St. Louis pushed across an unearned run to take the lead.

In the bottom of the inning, Tyrone Taylor moved automatic runner DJ Stewart to third base and Bader singled him home, bringing Vientos to the plate for his déjà vu moment. Initially, he wasn’t sure his shot off Liberatore would clear the fence, until he glanced at the scoreboard and saw the exit velocity flash at 105.8 mph. As Vientos returned his gaze to the field, Siani leaped and lost his glove in an effort to make a game-saving catch.

“I wish I could say what I said in the dugout,” quipped reliever Reed Garrett, who earned the win to join Jesse Orosco as the only Mets relievers with five in a calendar month. “It wouldn’t be good on live television.”

Whatever Garrett said was lost in the cacophony around Citi Field. It was loud and it was boisterous, and as the Mets poured out of the dugout to shower him with Gatorade, Vientos was not thinking about the fact that Marte will soon return. He wasn’t thinking about a return trip to Syracuse that might be inevitable regardless of how he performs. Others can, and certainly will, ponder those things for him.

Asked postgame about Vientos’ immediate future, Mendoza shrugged and smiled.

“We’ll see,” the manager said. “I’m pretty sure he’ll be in the lineup tomorrow.”