The injury happened on a fluke play late in the second quarter, when Williamson was the only defensive starter in the game.
Williamson and cornerback Tevaughn Campbell were in coverage in the end zone, defending running back Brian Hill. Williamson and Campbell collided, with Campbell diving into Williamson's knee as he tried to make a play on the ball.
It's a tough blow for the Jets because Williamson is one of their most consistent and durable players. He started every game last season, missing only five defensive snaps. The Jets figured to have a strong inside-linebacker tandem with Williamson and C.J. Mosley, one of their top free-agent additions.
It was a curious decision to have Williamson in a preseason game at that point, surrounded by 10 backups.
"Sometimes everybody is slotted for a different amount of snaps," coach Adam Gase said after the game. "Some guys, we're trying to get with certain packages. ... These last two weeks, he's kind of stayed in there a little bit longer. ... That's how we roll it. Some guys play a little longer than others."
Williamson, who began his career with the Tennessee Titans, signed a three-year, $22.5 million contract in 2018, including a $16 million guarantee. His $6 million salary this season is fully guaranteed. After that, there's no guaranteed money left in the contract.
He played middle linebacker last season, called the defensive signals and finished with 80 tackles, three sacks and one interception. He changed positions this season, moving to the weak-inside spot so Mosley could play the middle and call the signals.
The NFL Network first reported the ACL injury.