It happens this quickly. It happens in an eyeblink. Quicker than that, even. One moment you are covering a running back in the end zone, doing as your skills and your instincts have guided you to do your whole athletic life.

The next moment, the knee goes.

And the season follows.

So the Jets have lost Avery Williamson for the season now, the ACL in his right knee torn because a teammate named Tevaughn Campbell — also doing as his instincts and his skills have directed him to do as long as he knows — fell onto Williamson’s leg in a moment when the leg wasn’t in position to be fallen onto.

And we are reminded, again, just how fragile it all is, every bit of it: a football player, a football season, the delicate architecture of a team in a sport in which there are 17 different ways players can hurt themselves every time a ball is snapped.

We are reminded how what seems like a simple decision — keeping a front-line player in a meaningless game when most of the other front-line players have taken a seat for the night — can have profound ramifications. Yes, you can second-guess Gregg Williams and Adam Gase’s decision to have Williamson in there late in the second quarter when so many regulars had already called it a night.

But there is a value to playing, even in a meaningless game. The Falcons still had starters in the game at that point. The Falcons still had their franchise quarterback, Matt Ryan, in the game, so they were assuming a risk as well. And Ryan being out there meant there was value for Williamson, who had moved to weak-side linebacker this season, to be out there, too.

If only Campbell hadn’t fallen into his knee.

If only the ACL inside that knee hadn’t torn.

But these are the absurdities that football coaches have to make all the time at this time of year. They may work under the brightest glare in December and January, when playoff berths are determined and championships won, that may be when they are ultimately fired for failure or celebrated as geniuses.

But so much of what happens in December can be defined by what happens in August, when even passionate football fans have a hard time lasering their focus on exhibition games that the NFL deplores being called exhibition games because it underlines what they are: Exhibitions. Pretend games.

Pretend games with potentially damning consequences.

We’ve seen this around here before, of course. Jason Sehorn was on the precipice of superstardom in August 1998, coming off a breakout, breakthrough 1997 season, when he took the opening kickoff of the annual exhibition with the Jets and blew out his right knee. He was never the same.

One day shy of five years later, Chad Pennington — who’d attracted comparisons to no less than Joe Montana seven months earlier in leading the Jets to a 41-0 playoff win over Peyton Manning and the Colts — suffered a dislocated and broken wrist while throwing the ball away against the Giants. It was the first of a carousel of injuries that ensured Pennington would never quite max out the promise he’d hinted at.

And 10 years after that, Mark Sanchez suffered the final indignity of his Jets career when he hurt his shoulder during the fourth-quarter when Rex Ryan inexplicably and recklessly had him taking snaps during garbage time of the Snoopy Bowl.

“You can’t coach scared,” Bill Parcells, coaching the Jets, said the night Sehorn was hurt and he witnessed, first-hand, the nightmare of every coach working an exhibition game. “If you start protecting players, you’ll lose every game. It’s inconceivable to me not to use weapons when you have those weapons. This guy makes a difference in the game and you’ve got to use guys like that in the game.”

Easy to say. Harder to stomach. And it certainly makes you wonder just how carefully Gase must balance next week’s game against New Orleans, when he’d certainly love to see Sam Darnold go a full half against the Saints, when his curiosity would likely want to see Le’Veon Bell scamper a couple of times in green.

But at what cost?