Jets head coach Adam Gase admitted he made a bad decision.

In Thursday night’s preseason matchup with the Falcons, linebacker Avery Williamson was still playing late into the second quarter, while all other Jets starters were on the bench. Williamson was slow to get up after a play in the end zone where teammate Tevaughn Campbell collided with his right knee. An MRI exam later revealed the worst, a season-ending torn ACL for the 27-year-old Williamson.

Gase expressed his regret for leaving in one of his starters and said he told Williamson that.

“We had [Williamson] at a new position compared to what he’s played in the past. He and Neville [Hewitt] were together. The thought process was getting Neville’s feet wet, as far as playing ‘Mike’ linebacker, he’s playing a different position,” Gase said following Sunday’s Green and White practice at MetLife Stadium. “[We] wanted Avery to be in there, because Neville would be with a guy that’s a starter. Just looking back on it, hindsight 20/20, I wish I would’ve gotten him out of there a series earlier.

“I’ve talked with him. It’s on me. I’m the one that has to make that call and get him out of there and we didn’t. It’s a shame because he was having a good camp.”

Despite the season-ending injury, Gase says Williamson is “as positive as he can get right now.” The first-year Jets coach said he expects the former Titans linebacker to continue to guide the young players and to be a presence around the team.

The loss of Williamson, who is in the second year of a three-year, $22 million contract, is colossal. He led the team with 120 tackles last season with three sacks, an interception, two forced fumbles and a fumble recovery.

“It’s tough, man. He’s not only a great football player, but he’s a hell of a dude,” safety Jamal Adams said. “I feel bad for him. But obviously it’s the nature of the business — things happen. It’s hard to replace a guy like that. But, you know, obviously we have guys that are going to step up and going to learn the role and hopefully leave off where he left off, you know what I mean?

“At the end of the day, it is tough. We’re praying for him. He’ll be back better than ever.”

Williamson was beginning to get comfortable at weak-side linebacker after making the switch from Mike linebacker, which he primarily played last year. C.J. Mosley was brought on to become the new Mike linebacker, prompting Williamson’s switch.

In Williamson’s absence, Gase said he expects to use a variety of players in his place, with Hewitt leading the pack. Hewitt has had a strong training camp and, despite only starting four games last season, has proven reliable.

Linebackers Anthony Wint and James Burgess also could get an opportunity, with Burgess looking the most promising after a pick-six against the Falcons. Rookie Blake Cashman is also in the mix, but he is still working his way back from a leg injury suffered in the first preseason game against the Giants.

Gase officially guaranteed that running back Le’Veon Bell won’t see any preseason game time, admitting that Williamson’s injury solidified that decision.

Electing to withhold star players from preseason games is a common strategy, with the Giants setting an example with star running back Saquon Barkley. But Gase has now made the mistake once with Williamson, and clearly isn’t going to let it happen again.

“I was about 99.9 percent sure what we were going to do [with Bell during preseason]. That probably made it 100 percent,” Gase said.