Fantasy camp is over.

The last four-plus months have been like a dream sequence for Sam Darnold, the 21-year-old, even-keeled Californian who’s trying to assert himself as the first true long-term franchise Jets quarterback since Joe Namath.

The journey so far for Darnold, dating to the night he was drafted third overall by the Jets, has been a smooth one.

By all accounts, he’s mentally digested the playbook like a Rhodes scholar. He hasn’t had any physical ailments. He’s comported himself not unlike a 12-year veteran. He took first-team snaps in practice all summer and never blinked.

And most importantly, he’s performed confidently and admirably on the field in preseason games, looking and acting like he belongs.

That all led to the anticlimactic announcement that he would be the opening-night starter against the Lions on Monday night.

How smoothly Darnold’s journey proceeds from his dream summer to the reality of the NFL regular season will have as much to do with every player around him as it will have to do with him.

As Jets head coach Todd Bowles so perfectly put it Monday when officially naming Darnold the starter, the rookie will not be going it alone Monday night, there will be 52 other players on the team charter to Detroit.

Among those other 52, the players on the Jets defense will — and should — carry the particular burden of having Darnold’s back. The defense was better than the offense last year and it’s been improved in the offseason.

A fast, aggressive, disruptive start in Detroit by the Jets defense will make Darnold better immediately.

“Everybody’s going to have his back,’’ second-year safety Jamal Adams said Tuesday. “Sam is the ultimate competitor. He’s a humble guy. He comes to work every day and is a hell of a teammate. Everybody’s excited for Sam. We’re behind him. We’re going to support him through thick and thin.

“We understand that it’s a big stage, Monday night, for him. But there’s nobody’s hands we’d rather put that in than Sam’s hands.’’

As last year’s first-round draft pick, Adams offered this advice for Darnold in his NFL debut: “Stay composed. Adversity will happen. Stay composed and stick to the game plan that our coaches have taught us.

“Everybody is going to carry each other. It’s not about one man. It’s never going to be about one man. Everybody’s going to carry each other. Adversity is going to happen on defense. Adversity is going to happen on offense. If adversity happens on offense, the defense has got to pick [Sam] up.’’

Adams described this Jets defense like this: “We want to have that identity of guys that are going to fly around the ball and hit people in the mouth. That’s what we want to create. If we can put fear into an offense then we’ve got them before the ball is even snapped.’’

If the Jets defense can follow that blueprint delivered by Adams, then the rookie quarterback has a chance to play like a veteran.

“We’re not too worried about Sam,’’ defensive end Leonard Williams said. “That’s why he’s starting in the first place. It’s not too big for him. I’m sure it is a big moment, but I don’t think it’s going to be a big moment that gets to him. He can handle it. The most impressive thing is how poised and calm he is. He came in as a rookie and he seems like a professional already.’’

Jets’ 10th-year defensive tackle Steve McLendon said he doesn’t even view Darnold as a rookie.

“He’s a grown man now,’’ McLendon said. “We don’t have no pity for no boys. It’s a grown-man business. He knows what to do. He can spin the ball. He can put the ball in there. He knows his plays. I’m not going to look at him as no rookie. Once that ball is kicked off Monday night you’re no longer a rookie. When that ball is kicked off, you’re a vet.

“I don’t look at him any different than a 10-year vet. I look at him the same. I need him to go out there and perform.’’

Darnold can go out there and perform his 21-year-old butt off, but unless the defense is doing the same against the Lions, it won’t matter. That’s what makes Monday night in Detroit so real.