FLORHAM PARK, N.J. -- After two days of practice, New York Jets quarterback Sam Darnold declared himself ready to play after a bout with mononucleosis. At the same time, he acknowledged the potential danger of returning too soon.

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

While Darnold, his coaches and his teammates are optimistic about his chances of starting Sunday against the Philadelphia Eagles, the final call won't be made until Friday. He will 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. As of Monday, the day of his last round of tests, his spleen still had some swelling.

"I feel like I'm good enough to play," Darnold said. "My energy is great. I'm having fun out there. I'm having a good time in the huddle, breaking the huddle, just playing football. It's good to be back from that standpoint. I feel like I'm good to go, but obviously there are some things that need to get cleared before that happens."

Darnold was diagnosed in Week 2, but the Jets suspect he contracted the illness a few days before the season opener. He admitted he didn't feel well during the Week 1 loss to the Buffalo Bills.

If he receives medical clearance, it will be up to coach Adam Gase, general manager Joe Douglas, the trainers and the strength-and-conditioning staff to determine Darnold's availability for Sunday.

Gase made his preference clear: He wants Darnold behind center Sunday as long as the doctors sign off. If not, Luke Falk will start for the second straight game.

"It's all going to be about the test results," Gase said.

Darnold said he's preparing as if he will start, claiming, "Everything is clicking for me." He admitted, though, he's still ironing out some timing issues.

Gase said he's not concerned about Darnold's stamina and conditioning even though mono is an illness that drains the energy level. Sunday will be four weeks between games for Darnold, and he will have had only three practices to get ready. He didn't begin cardio work until this week and still hasn't been cleared to lift weights. He's getting most of the reps in practice, but there's a big difference between practice and game speed.

"There's a lot of communication going on with him," Gase said. "It's constant. We're checking with him: How are you feeling? Are you tired? What's your conditioning like? We keep talking to him. We keep watching him."

That the Jets have only two quarterbacks on the roster is an indication their confidence level in Darnold is high. If Darnold is ruled out, they'd have only Falk, meaning they'd have to elevate Mike White from the practice squad. White, signed last week, has no regular-season experience.

Theoretically, they could re-sign journeyman David Fales, whom they released 10 days ago after a one-week stint as the No. 2 quarterback. Fales knows Gase's system, but that would be an 11th-hour, desperation move.

All signs point to Darnold.

"If (the doctors) tell us he can go ... you can tell him not to play, but that's not going to go well," Gase said, smiling.

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.

"Hopefully, if the doctors give me the green light, there's no risk out there," Darnold said. "When I'm out there playing football, I won't be concerned about it."

In other injury news, left guard Kelechi Osemele suffered a potentially significant shoulder injury in practice. He likely would be replaced by Alex Lewis. Pro Bowl linebacker C.J. Mosley (groin) is expected to miss his third straight game.