One source called it a week-to-week injury, saying Darnold will be re-evaluated after the team's Week 11 bye. The Jets' first-round pick, who didn't practice Wednesday, watched from the sideline in a walking boot.
Officially, the Jets are saying it's day-to-day, refusing to rule him out Sunday against the Buffalo Bills, but no one actually believes he will play.
"We don't want to rush things," Darnold said after practice. "That's one thing we don't want to do. Yeah, I'm eager to get back out on the field with my guys, but at the same time, I have to take care of my body, first and foremost."
This was a mystery injury because Darnold never came out of last Sunday's loss to the Miami Dolphins, although he admitted afterward he was "dinged up." On Monday, he said he was dealing with "some bumps and bruises, but everything is good."
Coach Todd Bowles said he didn't become aware of the injury until Monday afternoon, about 24 hours after the Jets' third straight loss -- a game in which Darnold threw a career-high four interceptions.
"He had tests run," Bowles said. "I guess it got worse."
Backup Josh McCown, who has been mentoring Darnold, will start against the Bills (2-7). Davis Webb, currently on the practice squad, will be the No. 2 quarterback for the game after he's promoted to the 53-man roster.
Bowles said it's "too early" to say Darnold will be out, but he acknowledged it will be a "foregone conclusion" if he can't practice Thursday and Friday. The last thing the Jets want to do is expose their potential franchise quarterback to further injury. Bowles said the looming bye week will factor into the decision, meaning the Jets will err on the side of caution.
Bowles said it's not a Lisfranc injury, which is good news. Asked if surgery is a possibility, Bowles said, "Not at this time. That's what I was told [by the medical staff]."
Until Wednesday, Darnold never had missed a practice. On Sunday, he got up limping after a third-quarter sack by Cameron Wake, although he said he wasn't sure when the injury occurred.
"I thought I was fine coming out of the game," he said. "I went through the protocol. I went through some tests, and obviously something was a little different."
The timing is lousy for Darnold, who is mired in a slump. He has seven interceptions in the past three games, bringing his total to a league-leading 14. He said he played "stupid football" against the Dolphins.
On Monday, Bowles said he had no plans to bench Darnold. The injury changes the plan.
"It's adversity," said Darnold, who ranks next-to-last in passer rating (68.3). "You just have to deal with it, get better and get on the field as soon as possible."
McCown, who started 13 games last season for the Jets, will be rusty. The Jets were so focused on Darnold and Teddy Bridgewater (who has since been traded) in the preseason that McCown barely played -- he took only 19 snaps.
"I do enough every week [in practice] to kind of keep myself tuned up," the 39-year-old journeyman said.
Still, it will be difficult. McCown hasn't worked much with wide receivers Robby Anderson and Quincy Enunwa, both of whom didn't practice with ankle injuries. They're expected to play, but their practice time will be limited.
"Any time you get an injury to a starter, it's a blow to the team, but Josh has been practicing," Bowles said. "He's been the guy. If he has to play, we're confident with him."