Sam Darnold's heart said yes, but his spleen said no.

The New York Jets quarterback, who declared himself "good to go" on Thursday, was ruled out Friday after lab tests revealed an enlarged spleen -- the residual effect from a bout with mononucleosis.

It's another unexpected turn in the Jets' hard-luck season. They will be forced to start Luke Falk on Sunday against the Philadelphia Eagles.

"The labs didn't come back the way we were hoping," coach Adam Gase said. "I know Sam is disappointed, but we'll be able to adjust and work quickly."

This puts the Jets (0-3) in a bind because Darnold received the majority of the reps in practice on Wednesday and Thursday. He was optimistic about his chances of playing, as were his teammates and Gase, but he didn't receive medical clearance.

Darnold, who thought he'd be tested after Friday's practice, actually went for an ultrasound and blood tests on Thursday night. The results revealed swelling in his spleen -- a potentially dangerous situation, as an enlarged spleen could rupture with a blow to the midsection.

"It's close, but it's not where it needs to be," Gase said.

Falk said he learned Thursday night that he'd be starting, and he took all the reps on Friday. His initial reaction?

"Sad for Sam that he has to go through this another week, but I'm excited for my opportunity," Falk said.

Gase stopped short of saying he expects Darnold to play next week against the Dallas Cowboys, saying, "We'll see." Gase said he's hopeful.

This will be the third straight game without Darnold, who was ill for the season opener and diagnosed with mono two days later. Trevor Siemian started Week 2, but he suffered a season-ending ankle injury. Falk started Week 3.

"He tried to do everything he could," Gase said of Darnold. "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."

Gase said it was "a good call" by the doctors, although he admitted they weren't anticipating this outcome.

Scrambling for a backup, the Jets re-signed journeyman David Fales, who served as the No. 2 quarterback in Week 3. They cut him during the bye week, thinking Darnold would be active. The other option was activating Mike White -- signed last week to the practice squad -- but he has no regular-season experience.

Gase tried to put a positive spin on it, saying he could sense a rallying spirit among the players, but this was a major blow on many levels.

On Wednesday and Thursday, Darnold's teammates spoke excitedly about how they looked forward to his return. Running back Le'Veon Bell said, "I'm happy as hell that he's back." Darnold was confident, insisting he wasn't concerned about his condition. A week ago, he received a custom-made, hard-plastic brace to protect his spleen.

Then came the lab results.

The Jets' offense has struggled mightily, having produced one touchdown in three games. Falk led the offense to a field goal after replacing Siemian in the middle of a possession in Week 2, but he has gone 21 straight drives without points since.

"Luke is wired in," Gase said.