The dip in velocity was noticeable. Pitches that were coming in at 93 or 94 mph in the first inning deflated to 91 and 92 as Jacob deGrom labored in the fifth. Solid contact seemed inevitable, and so did the pair of runs the Marlins scored in that inning.

And then the final indignity: DeGrom, in distress, signaling for the trainer as he walked off the mound, his night over and his health under scrutiny.

“It’s a little troubling for me,” manager Terry Collins said of deGrom’s visit with the trainer, which he learned about only after the Mets’ 6-4 loss to the Marlins on Thursday night at Citi Field. “That’s not Jake, and that’s the thing that we’ve been watching. His command is not what it has been, and that’s the big issue.”

DeGrom said he’s fine — physically, at least. He said he was “out of sync” and “frustrated” and a few other troubling descriptions.

“I didn’t feel great out there today,” deGrom said, noting that the problem is mechanical in that he’s falling too far off to the first-base side with his landing leg. “I walked four guys. I can’t put the ball where I want right now. I’ve got to figure that out.”

It’s been a tough two weeks for deGrom, and what briefly looked to be a return to form by the Mets took an unwelcome turn.They fell two games behind the Cardinals for the second wild-card spot and are down a second baseman, as Neil Walker confirmed that he’ll undergo back surgery to deal with a herniated disc.

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In addition to the psychological blow, deGrom’s unsettling performance put the Mets in a hole from which they couldn’t emerge, though they got close.

Jay Bruce, mired in a horrendous slump and routinely booed when he comes to the plate, slammed his 28th home run in the sixth, a solo shot to center that drew the Mets to within 3-1. Bruce, who came into the game batting .183 since joining the Mets, tacked on an RBI single in the eighth.

The Mets loaded the bases with none out in the latter inning, but Michael Conforto hit into a 1-2-3 double play and pinch hitter Yoenis Cespedes struck out swinging. Pinch hitter Asdrubal Cabrera slammed a two-run homer in the ninth.

Things began to unravel in the fourth for deGrom, who was pitching on seven days’ rest. He didn’t give up any runs but allowed a hard-hit single by Jeff Mathis and a loud out to center by Dee Gordon.

The downward trend continued in the fifth after he recorded two quick outs. Christian Yelich (three hits, four RBIs) singled and stole second before Jeff Francoeur and Xavier Scruggs made it 3-0 with RBI doubles. Mathis struck out, but deGrom was done after 102 pitches, having given up six hits and four walks with six strikeouts.

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The Marlins made it 6-1 in the seventh on Yelich’s three-run homer off Josh Smoker.

Even with deGrom saying he’s healthy, the specter of this start appeared to weigh heavily on Collins, who already has dealt with injuries to Matt Harvey and Steven Matz.

“Jake deGrom is a huge piece,” he said. “You can’t lose three-fifths of your rotation, coming out of spring training, arguably as good as there was in the major leagues. You can’t lose three-fifths and still be in good shape.”