ST. LOUIS — Brandon Nimmo said he first felt shortness of breath after collecting three hits on the Fourth of July. He likened the feeling in his chest to the tightness that comes after breathing in cold air.

For a few days, the Mets outfielder thought little of it. But on Friday, he mentioned the lingering sensation to the team trainers, who sent him to the hospital for tests. The diagnosis — a partially collapsed lung — landed him on the 10-day disabled list Saturday.

“When they said it was a collapsed lung, I was taken aback, too,” he said.

Nimmo and the Mets believe he should be back in action shortly after the All-Star break. The DL move is retroactive to Wednesday, meaning Nimmo is eligible to return July 15.

“It sounds a lot more serious than it probably is, as far as my case is,” Nimmo said. “But when you hear collapsed lung, I did not think that was what we were going to hear.”

Outfielder Michael Conforto was activated from the disabled list, though he did not start Saturday against the Cardinals. Conforto, the Mets’ lone All-Star representative, had been on the shelf with a bruised left hand after being struck by a pitch.

Nimmo, 24, is hitting .350 in 15 games since his promotion from Triple-A Las Vegas. He already has missed time this season with a hamstring injury and a nerve issue in his thumb.

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Nimmo missed Saturday’s game because he was taken to a hospital to be diagnosed. Collapsed lungs can afflict athletes, particularly those who are tall and lean, like Nimmo.

“I’m only stating what the doctor said,” manager Terry Collins said. “It’s really not as serious as it sounds . . . There’s no real cause. It’s just one of those things that sometimes happens in athletes that are in great shape.”

Nimmo planned to spend the All-Star break in Nashville, but because doctors have prohibited him from flying, he’ll make the 4 1⁄2-hour drive instead. He hopes to be cleared to fly by Thursday.