Sam Darnold may think he’s ready to return, but the doctors do not.

So the Jets will be without their starting quarterback for the third straight game.

Adam Gase said Darnold, who was diagnosed with mononucleosis after Week 1, was ruled out by doctors on Thursday night after further testing to see if his body had fully recovered enough to face the Eagles on Sunday. The tests did not come back the way the Jets were hoping.

Luke Falk will now get the start in Philadelphia.

“I know Sam’s disappointed, but we’re able to adjust and work quickly,” Gase said before practice Friday. “Our guys have done a good job with any kind of changes we’ve had to make.”

The hope was Darnold would be cleared to play after he practiced earlier in the week. That did not happen.

Darnold’s spleen remains enlarged and he won’t be cleared for contact until it returns to normal size.

The 22-year-old has been practicing with a custom-made, thick-plastic brace to protect his rib-cage area. He intends to wear the brace — which is supposed to prevent his spleen from rupturing if he is hit in the midsection — when he first resumes playing.

The initial plan was for Darnold to undergo tests on Friday, but doctors suggested moving the tests up to Thursday night.

“I felt bad for him,” Gase said. “When we told him, the first thing he says is, ‘I feel like I’m letting the other guys down.’ It’s not about him. He tried to do everything he could. He did everything he was asked to do. He did everything the doctors told him to do. It just didn’t work out for him.”

The Jets re-signed David Fales on Friday to serve as Falk’s backup. Fales was the backup in the Week 3 loss to the Patriots, but was released last week amid speculation Darnold would be ready to play. Fales, a sixth-round pick of the Bears in 2014, was with Gase in his past two seasons in Miami.

Darnold saw a bulk of the first-team reps this week in practice, but Gase isn’t concerned with Falk’s preparedness. Gase said Falk also practiced with the first team and had additional reps with the scout team.

It’ll be the second start for Falk, who was claimed off waivers from the Dolphins in May.

“We were ready for either way,” Gase said. “We didn’t know. Even when we started it, we didn’t know. We were just trying to make sure than anything that we did was the right thing as far as play calls and things like that to make sure we didn’t put him in a bad spot if they cleared him.”

Falk was able to get a full practice with the first team Friday. The 24-year-old Washington State product expressed his concern for Darnold, but is looking forward to another opportunity

“I think just having the first start under your belt. Kind of that game action, how you go about the week,” Falk said of how his start against the Patriots will help him Sunday. “How you go about the flow as a starter. Some good learning lessons there.”

After losing Darnold so early in the season, the Jets’ offense has foundered, failing to reach the end zone in two games. And with guard Kelechi Osemele missing practice with a shoulder injury — on top of the knee issues he was already having — matters have only gotten worse for the Jets’ struggling offensive line.