ST. LOUIS -- Cardinals manager Oliver Marmol can handle closer Giovanny Gallegos spiking a couple of curveballs when he had the Mets down to their final strike in the bottom of the ninth inning. And he can even handle the usually reliable stopper surrendering three hits that unexpectedly reduced St. Louis' lead from two runs to one.

But Marmol said what can’t be tolerated are the mental mistakes that played major roles in a two-run St. Louis lead devolving into a 5-2 loss to the Mets in a downright unsightly ninth inning before 35,455 stunned fans at Busch Stadium on Monday.

As if giving up four hits in the ninth wasn’t indicative enough of Gallegos' struggles on the night, he failed to cover first base in time to beat Mets pinch-hitter Dominic Smith to the base. Then, matters got worse when a stunned Gallegos failed to react fast enough to what was happening, allowing the tying and go-ahead runs to score on an infield grounder that could have ended the game.

“Pretty simple -- we’re going to make physical mistakes, but that was a mental mistake, and we can’t excuse it,” Marmol said bluntly about Gallegos' first slipup of the season. “He knows it, we know it. He’s got to cover first.”

Within a terrible 12-minute stretch, the Cardinals wasted an eighth-inning leadoff single from veteran catcher Yadier Molina, a potential winning run from rookie Brendan Donovan -- who was making his MLB debut -- and a masterful pitching performance from Miles Mikolas. The Cards righty proved himself to be every bit the equal of his Jupiter, Fla., offseason workout buddy, Max Scherzer, on Monday night. Whereas Scherzer struck out 10 over seven scoreless, Mikolas allowed just four hits and struck out five over seven scoreless innings of his own. That work allowed Mikolas to extend his scoreless streak to 12 innings, and the 6-foot-4 veteran has allowed just one run over the past 18 2/3 innings.

“There’s always pressure when you’ve got a good opponent on the mound, but I started first and he had to match my zeroes,” said Mikolas, who saw his ERA dip to 1.21 after his strong outing versus Scherzer, who recorded the 106th double-digit-strikeout performance of his stellar career.

That it was Gallegos who surrendered the lead was surprising to the Cardinals because the 30-year-old has been so unflappable for the club for most of the past two seasons. Last season, Gallegos helped save the Cards' season when closer Alex Reyes struggled with his command and had right shoulder issues late in the season. The native of Sonora, Mexico, came on to record 14 saves to go with his 24 holds. This year, Gallegos had closed out his first four save opportunities without a hiccup.

On Monday, Gallegos retired Pete Alonso to start the ninth, but allowed Eduardo Escobar to reach on a single. A batter later, he got Robinson Canó to pop out to left field and Gallegos got two quick strikes on Mark Canha. Gallegos threw five pitches with a chance to close out the game before Canha grounded sharply to third base to Nolan Arenado. With the game in his hands, the nine-time Gold Glover threw from foul territory uncharacteristically high of first base.

“That’s a tough play, he’s on the run and he overthrows [first base],” Marmol said. “He’s going to make that play more often than not. That time, it didn’t happen.”

From there, Gallegos allowed Jeff McNeil to double to put the tying and go-ahead runs in scoring position. For an instant, it looked as if the Cardinals' closer would get out of the jam when he got Smith to ground sharply to first and the ball was snagged by Gold Glover Paul Goldschmidt. The issue came when Gallegos didn’t make it to the bag in time to beat a hustling Smith. Making matters worse, McNeil, following Travis Jankowski, came around and scored from second before Gallegos could turn and throw home.

“It was hard contact, and it didn’t work as he wanted it to work,” Gallegos said through bullpen catcher/interpreter Kleininger Teran. “The second [Smith] hit it, I thought it was a foul ball, but the ball bounced off first and that’s when he reacted. But I am going to keep working and trying to get better for next time.”

Off balance from getting to the bag late and stunned by the series of events, Gallegos fired home late as McNeil scored the game’s go-ahead run.

“After I stepped on the base, I didn’t have the good balance to throw the ball to home plate, so that’s why it took so long,” Gallegos said. “Couple of ground balls, but it didn’t work. But I know our defense is one of the best and they are always there to help. It’s a hard night, but I will get better from this.”