Forget about game plans. Ignore preparation and practice. The key to the Jets’ first victory of the season might be as simple as a visit to the doctor’s office.

That is what will determine whether they get starting quarterback Sam Darnold back for Sunday’s game at Philadelphia.

The second-year signal-caller believes it will be his final hurdle as he looks to return from a bout with mononucleosis that has cost him the past two games. He will get an ultrasound on his enlarged spleen and undergo blood tests Friday, and if cleared medically, the face of the franchise expects to be back under center as the Jets hope to avoid an 0-4 start.

“That would be the case, if everything’s normal and we’re good to go,” said Darnold, who was diagnosed with mono just over three weeks ago, adding later, “Right now, I’m preparing and I’m practicing like I’m going to be the starter.”

Darnold has practiced the past two days in a red no-contact jersey and said he feels fine, not tired or fatigued. Teammates and coaches have echoed a similar sentiment, saying there has been little rust or physical limitations they’ve noticed from their quarterback, who has taken almost all of the first-team reps the past two days. Custom-made padding has been designed to protect his spleen.

“The ball’s been coming out clean, they’re accurate, they’re on time,” running back Le’Veon Bell said following practice Thursday. “He looks like the same guy.”

Still, it remains uncertain whether Darnold will get cleared to play. He said he feels normal, but there is no telling if his spleen is still enlarged until he sees the doctor again Friday. If there is a chance of him getting injured because he isn’t fully recovered, he will not be put out there.

“I want to make sure I’m safe out there and I’m not going to die, I think that’s one thing,” Darnold said with an uneasy chuckle. “But for me sitting back and watching the team play, and not having anything to do with a win or a loss, it’s not a good feeling for me. If there’s one thing I realized this time sitting is that it’s a privilege to play this game.”

The Jets have missed him dearly, producing three points on offense the past two weeks in ugly losses to the Browns and Patriots. Luke Falk, who began the year on the practice squad, has been asked to lead the offense after Trevor Siemian was lost to a season-ending ankle injury in Week 2.

Getting Darnold back would be a huge boost for an offense coach Adam Gase said has performed “terribly.” Bell admitted “it was sickening” watching film.

“I’m happy as hell he’s back,” Bell said. “Me playing the running back position, I hope he’s out there.”

Added offensive coordinator Dowell Loggains: “Any time you put your franchise quarterback, the guy that [everybody in that] locker room believes in and knows how talented he is, and you drop him in the starting lineup, I think it lifts the morale of the whole team. It lifts the morale of the coaching staff, the fan base.

“That guy’s not replaceable. It does excite the team. It gets the guys fired up. Not that they weren’t excited to play with Luke or Trevor, but when you get your starter back, everybody knows who the face of the franchise is.”

If Friday’s checkup goes well, and his spleen is no longer enlarged, then the expectation is Darnold will play against the Eagles. The Jets will confer with everyone, from Gase to general manager Joe Douglas, Darnold, doctors and trainers before anything is decided, Gase said. Everyone will have to sign off on his return.

“If they tell us he can go, you can tell him not to play,” Gase said with a smile. “It’s not going to go well.”