ST. LOUIS — The sign from behind the plate was clear, so Mets ace Jacob deGrom brought his hands together and felt for the proper grip on the baseball. He began his long, flowing motion, flipping a curveball to the Cardinals’ Stephen Piscotty.

It loitered in the wrong neighborhood, no different from most of deGrom’s offerings in his last two starts. Piscotty reached down and golfed the curveball, depositing it over the fence in leftfield for a fifth inning home run.

A moment later, deGrom dropped his head when the drive reached the seats, giving the Mets their latest reason to sound the alarm.

“I feel fine,” deGrom said after getting drubbed in Wednesday’s 8-1 loss to the Cardinals. “It’s hard to get results when you throw everything down the middle.”

With the Mets scratching and clawing to maintain their slim hopes of sneaking into the playoffs, deGrom got throttled for five runs and 12 hits in just 4 2⁄3 innings. He tied a career worst by allowing three homers, by Matt Carpenter, Randal Grichuk and Piscotty.

His previous start was a stinker against the Giants, when he set career highs in hits allowed (13) and runs (8). The tally for deGrom’s last two starts: 13 runs, 25 hits, 9 2⁄3 innings.

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Still, with deGrom on the mound, the Mets (63-63) had reason to believe that they might gain one more game in the race for the final playoff spot. They gave one back instead, falling 4 1⁄2 games behind the Cardinals (67-58), who maintain possession of the second wild card.

“Obviously, when you get hit like he’s been hit it’s lack of command,” Mets manager Terry Collins said.

The outing raised more red flags about a pitching staff that has shown signs of wilting under the summer heat. The Mets entered Wednesday with a 6.09 ERA during a 10-game trip that wraps up Thursday when the Cardinals’ Adam Wainwright (9-7, 4.71 ERA) takes on Seth Lugo (0-2, 3.04 ERA).

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In the afterglow of winning the pennant, manager Collins heard warnings last winter about the price that might be paid by a young staff that logged innings deep into October. Giants counterpart Bruce Bochy coined it the World Series hangover.

“I didn’t know what he meant, to be honest,” Collins said recently.

Now, he knows.

With the Mets entering the critical home stretch of their season, their vaunted young rotation has been crushed by attrition.

For fear of taxing his arms, Collins refused to use Monday’s day off as a way to skip Niese in the rotation because it would have come at the expense of extra rest for the likes of deGrom.

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Preventative measures, however, have not spared the righty. Collins wondered if deGrom might benefit from more rest, though the pitcher brushed off thoughts of physical concerns.

Said Collins: “At this stage of the season, the first think you look at is fatigue.”

Early in the season, the righthander worked through waning velocity before settling into a groove. Even after his worst start of the season, deGrom looked forward to facing the Cardinals. He was even greeted with humidity on a 90-degree night, the kind of weather he grew comfortable with growing up in Florida.

The conditions did not help, neither did pitching on extra rest.

Trouble arrived with the first batter in the first inning. With the count full to Carpenter, deGrom challenged him with a belt-high fastball over the heart of the plate. It landed in the Cardinals’ bullpen.

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By night’s end, the Mets were considering whether extra time might benefit deGrom. But with a staff depleted by injuries, such a thing might be nothing more than a luxury.

Said Collins: “We’re running out of bodies.”