ST. LOUIS — Pete Alonso didn’t realize his mistake until he heard voices from the Cardinals’ dugout yelling at him.

After rookie Masyn Winn had reached on an infield single in the fifth inning Friday, Alonso said he was told by first base umpire John Tumpane a new ball was needed.

Alonso, as is his habit, flipped the old ball into the stands.

But in this instance Winn, who had been promoted from Triple-A earlier in the day, had just gotten his first major league hit. And the ball was now in the possession of a fan.

“I feel awful,” Alonso said after the Mets’ 7-1 victory at Busch Stadium. “I know it sounds stupid, but it’s just a bad brain fart. … I feel like a piece of crap. In the heat of the moment you just kind of get lost.”

Alonso was relieved after the game to hear that the fan eventually surrendered the ball, giving Winn his memento.

He said he apologized to Winn at second base and plans to bring him a gift on Saturday.

“I’m happy he’s got the ball, but I feel like an idiot,” said Alonso, who was heavily booed during his ensuing two plate appearances. “That was a complete bonehead move and I feel awful.”

Alonso said former WBC teammates Nolan Arenado and Paul Goldschmidt yelled, “Pete, what the hell are you doing?” after he threw the ball into the stands.

“I’ll never throw a ball into the stands again — I will just roll every ball into the dugout when they are going to switch it out,” Alonso said.

Trevor Gott’s tenure with the Mets began in unspectacular fashion, but the right-handed reliever has emerged as a dependable pitcher in recent weeks.

That has included closing games. With Adam Ottavino and Brooks Raley unavailable Thursday because of usage, Gott was thrust into the ninth inning to protect a two-run lead. Two runners reached base, but Gott survived by striking out Goldschmidt to end the game and earn his first save with the club.

“He’s a guy that is capable of pitching in a lot of spots and as long as you keep him to one-inning stints he’s pretty durable,” manager Buck Showalter said of Gott, who did not pitch in the Mets’ win on Friday night. “I think it took us a while to understand what we had and put him in a position to succeed.”

Gott has been unscored upon over his last 12 appearances, spanning 11 ¹/₃ innings. The Mets acquired him in a July 3 trade with the Mariners that also yielded Chris Flexen (who was immediately released with the remainder of his $8 million salary for this season absorbed).

Gott, who is under club control through next season, pitched to an 11.57 ERA in his first seven appearances with the Mets, which included a blown save against the Dodgers on July 16.

“I think he’s gotten better with left-handed hitters,” Showalter said. “He’s a durable guy if you don’t overuse him in a given outing.”

Grant Hartwig was optioned to Triple-A Syracuse to create space for starting pitcher Joey Lucchesi. The move left the Mets with seven relievers, but Showalter said there were contingency plans in place to get extra bullpen help as soon as Saturday.

The rookie Hartwig had consecutive rough appearances, against the Braves and Pirates, before recovering with a scoreless inning against the Cardinals on Thursday.

“His innings and pitch [total] are in good shape, so I am not worried about that right now,” Showalter said. “It’s always good for a young pitcher to take a blow here and there, but he has presented himself pretty well.”

Keith Hernandez will participate in the Cardinals’ Hall of Fame induction ceremony on Sunday.

The Cardinals are inducting Max Lanier and Jose Oquendo. Hernandez was inducted into the Cardinals Hall of Fame in 2021.