Carlos Mendoza was seen in the dugout scratching his ear and subtly grimacing as the Marlins’ Xavier Edwards hit a run-scoring single. It was the fourth run in a disastrous six-run frame for the Mets.

“You might take your whole travel ball squad off the field if something like that happened,” SNY announcer Ron Darling said in the moment. Darling said the quiet part out loud: What’s going to happen if the Mets play an inning like this in October?

With the way the Mets are playing — and the way they’ve played over the last couple months — there might not even be an October.

Gary Cohen called it a “slow-motion, backwards march.”

This march is now reaching its final few counts this weekend.

Plenty of issues have plagued the Mets’ season, from the starters not going deep to the merry-go-round that is the bullpen. But the one that continuously stings them, perhaps the harshest of all, is their lack of tight fundamentals in the biggest spots.

The Mets’ defense overall is fine; they’re middle in the pack in defensive runs saved, with their 23 DRS (defensive runs saved) ranking 15th in baseball as of Friday. But this is less about their defense in general, and more about doing the little things. Fielding ground balls. Covering bags. Making accurate throws.

They didn’t do those things enough in the fifth inning on Friday, and it ballooned the game out of proportion.

First, there was the Pete Alonso play. Alonso’s bat has been a driving force this season, but his first-base defense continues to be all over the place. To his credit, he handled a hard liner for the first out. But one batter later, with Brandon Sproat still in the game, he booted one off his glove.

Had he fielded it cleanly, he would have had a chance to get Troy Johnston at the plate. Instead, Jeff McNeil had to shuffle it back to him, settling for the out at first base.

It was the go-ahead run.

Mendoza had a quick hook on Sproat there after just 61 pitches. He’d been lights-out through the first four innings. While this can’t just be pinned on Mendoza, he has made a boatload of mid-game decisions throughout the season that haven’t worked out. Sometimes it’s leaving the starter in one batter too long, and sometimes it’s taking him out one batter too early. Either way, things have often gone awry for the Mets in the middle innings.

And Gregory Soto has been in the thick of that on more than one occasion.

He entered this time with two outs and Agustín Ramírez on first base. The Mets had lost the lead, but Soto still just needed one out to escape with it being a one-run game.

That didn’t happen.

Ramírez not only stole second, but stole it wildly early without a throw. He then stole third as third baseman Ronny Mauricio — in for an injured Brett Baty — failed to cover third base. The Mets essentially gifted Ramírez two free bases.

Edwards promptly lined one up the middle. It cost the Mets a run right there.

But somehow, the inning still wasn’t over. Pinch-hitter Connor Norby prolonged Soto’s nightmare with a two-run bomb to blow it open. Soto threw 20 pitches before he finally got the single out he needed.

A discussion could be had here about the Mets’ bullpen issues, and the lack of length from their starters, both of which played a factor in this game. But the lack of fundamentals lingers, especially with how many times it’s come up this season. Its significance is only exacerbated given the current standings situation.

Mendoza was asked after the game about why they haven’t been able to clean things up.

“It’s on me. It’s on all of us,” he said. “We continue to make the same mistakes and it’s costing us games.”

Now, it might have just cost them one too many. They still have a chance — but the Reds could eliminate them by simply winning two more games. The Mets need to finish a game ahead of the Reds by the end of Sunday to squeak into the postseason fold.