Darnold, diagnosed four weeks ago with mononucleosis, had been dealing with an enlarged spleen -- a residual effect of the illness. He said he felt well enough to play last week, but there was still some swelling in his spleen, the team said.
Darnold, speaking Monday afternoon in his weekly radio spot on The Michael Kay Show on ESPN New York, said: "I feel good. Energy is awesome. Just looking forward to playing this week, but have to wait for doctor's orders."
His return should provide an emotional boost for the reeling Jets (0-4), who have scored only two touchdowns on offense. The tension in the locker room prompted a players-only meeting on Monday. Players said they called the meeting in an attempt to remain unified.
Also returning is tight end Chris Herndon, who served a four-game suspension for violating the league's substance-abuse policy. Herndon hasn't yet been added to the roster; he has an exemption.
The good news for Darnold is that he had two full days of practice last week. He and the team were so confident he'd be cleared that he worked exclusively with the starters on Wednesday and Thursday. Late Thursday, he was ruled out after an ultrasound and blood test.
Scrambling, the Jets had to start Luke Falk for the second straight week. He was manhandled in a 31-6 loss to the Philadelphia Eagles. He was sacked nine times and committed three turnovers, two of which were returned for touchdowns.
Fearing a concussion, the Jets pulled Falk after his ninth sack. They inserted David Fales, their fourth quarterback of the season. Falk cleared concussion protocol, the team said, and is available this week.
The Jets, off to their worst start since 2003, hope Darnold can galvanize a punchless and mistake-prone offense.
"It's probably more than just Sam getting out there -- we've got a lot of other things to clean up -- but Sam is able to hide things sometimes for us," Gase said. "If we make a mistake, he covers it up. We just need to do a better job as a group, making sure we're all on the same page."