Mets manager Terry Collins said Nimmo was complaining of some chest issues on Friday.
"We were lucky we had a doctor on site here," said Collins, "and [the medical personnel] gave him an examination and told him they needed to take him to the hospital and run some tests -- and that's how they discovered what it was."
With Nimmo's move to the DL, the Mets reinstated outfielder Michael Conforto, who was dealing with a left wrist contusion.
Nimmo isn't expected to miss a significant amount of time, however.
"I think because it's such an important part of the body ... this is just something that we should be conservative about," Nimmo said. "But I really don't expect [to be out] too long -- we're not talking months. I don't think we're talking anything like that. It's obviously already healed or else air would still be leaking into my chest cavity."
Collins was asked what caused Nimmo's collapsed lung, but apparently there was nothing specific -- like a collision -- that caused the condition.
"I asked this morning [what caused the issue], obviously knowing that I was going to be asked what the cause was -- and [the medical people] said there's no real cause, it's just one of those things that sometimes happens to athletes who are in great shape," Collins said. "He's gonna have another test done [on Sunday]. Hopefully, they can get it taken care of and we'll let him rest for a few days and then get him back on the field."
Collins said the issue isn't as concerning as it sounds.
"I'm only stating what the doctor said, but it's really not as serious as it sounds," Collins said. "A collapsed lung is a collapsed lung, but it's really not a serious, serious thing -- and they think they can take care of it and he'll be fine."
Nimmo, the Mets' first-round Draft pick (No. 11 overall) in 2011, appeared in 15 games this season and owns a .350/.458/.400 line with two RBIs in 24 plate appearances.
Collins said he expects Nimmo to be available to return to action not long after the All-Star break.
"Obviously, he hasn't played since Tuesday, so I don't know how far we can go back," said Collins. "But I think he'll miss this week and [he] should be OK a couple days after we start back up."