League sources told ESPN and NFL Media on Friday that Williamson is expected to miss the entire season due to the torn ligament in his right knee. He had an MRI Friday that confirmed the severity of the injury.
In the second quarter of Thursday night's preseason contest against the Falcons, Williamson, who was the only defensive starter remaining in the game, collided with teammate Tevaughn Campbell while in coverage. Campbell accidentally made contact with Williamson's knee as he tried to make a play on the football.
Williamson started all 16 regular-season games last season, missing only five defensive snaps. He recorded a career-high 120 combined tackles, three sacks, two forced fumbles, one interception and six pass breakups in 2018, his first season with the Jets.
The Tennessee Titans selected Williamson in the fifth round of the 2014 NFL Draft out of Kentucky. He spent four seasons with the Titans before joining the Jets on a three-year, $22.5 million contract.
Williamson was expected to form an inside-linebacker tandem with top free-agent acquisition C.J. Mosley. Neville Hewitt and Anthony Wint are leading candidates to replace Williamson in the lineup.