Despite declaring himself ready, quarterback Sam Darnold will not return for the New York Jets this week, coach Adam Gase said Friday.

Luke Falk will start for the second straight game when the Jets face the Philadelphia Eagles on Sunday.

Darnold has not played since Week 1 after being diagnosed with mononucleosis on Sept. 12.

He was not cleared to play after undergoing testing Thursday night, Gase said. Darnold was set to have an ultrasound and a blood test to determine if his spleen, previously enlarged, has returned to its normal size.

An enlarged spleen -- a common symptom of mono -- could rupture with a blow to the midsection. Darnold has been practicing with a custom-made, hard-plastic brace that protects his rib-cage area and he's planning to wear the brace when he plays.

Darnold had said he felt good enough to play but wouldn't risk his long-term health.

"It's a tough thing because you want to stay safe," Darnold said Thursday. "I want to make sure that I'm safe out there and I'm not going to die."

The Jets (0-3), coming off their bye week, are desperate for a spark. The offense has generated only one touchdown, having gone 29 straight possessions without reaching the end zone. They have started three different quarterbacks -- Darnold, Trevor Siemian and Falk. Siemian suffered a season-ending ankle injury in his first game.

ESPN's Rich Cimini contributed to this report.