The Jets’ defense has suffered a significant blow.

An MRI exam Friday revealed a torn ACL in the right knee of starting linebacker Avery Williamson, according to a source. Williamson will miss the entire 2019 season.

The injury occurred toward the end of the second quarter of Thursday night’s preseason game at Atlanta. Williamson was covering Falcons running back Brian Hill in the end zone. Cornerback Tevaughn Campbell dove to try to deflect a Matt Ryan pass and hit Williamson in the right knee.

The decision that led to Williamson still being in the game at that point is a curious one. He was the only veteran starter still playing in the game for the Jets. It likely was defensive coordinator Gregg Williams’ decision to keep Williamson in the game.

“Everybody’s slotted for a different amount of snaps,” head coach Adam Gase explained Thursday night. “Some guys we’re trying to get with certain packages and you’re a 1 in this group, a 2 in this group. The last two weeks, he’s kind of stayed in there a little longer.”

Gase did not speak with the media on Friday and is not scheduled to do so again until Sunday.

Williamson is in the second year of a three-year, $22 million contract with the Jets. He led the team with 120 tackles last year after arriving as a free agent following four seasons with the Titans. He also had three sacks, an interception, two forced fumbles and a fumble recovery.

Williamson, 27, was in the middle of changing positions. He was moving to weak-side linebacker this year after playing the “Mike” linebacker last year. The team signed C.J. Mosley to a five-year, $85 million deal to be the new Mike linebacker, pushing Williamson over. He had been embracing the move, though. Earlier in the second quarter, Williamson sacked Ryan.

The Jets now must find his replacement. The first candidate likely will be Neville Hewitt. The 26-year-old started four games for the Jets last year, when Darron Lee was suspended, and he played well. Hewitt has followed that up with a strong training camp and preseason this year.

Hewitt is a very physical player who is not afraid to dish out big hits and is around the ball quite a bit. The question will be how well he can cover running backs and tight ends, something Williamson also struggled with.

The other linebackers on the roster are rookie Blake Cashman, Anthony Wint and James Burgess. Cashman has been sitting out with a leg injury since the team’s first preseason game against the Giants last week. Wint was with the Jets last year, playing in two games. Burgess had an interception Thursday that he returned for a touchdown. Burgess played in 17 games over the past two years for the Browns.

The Jets entered training camp knowing they could not afford too many injuries. The team feels good about its starting lineup, but depth is a major issue. Cornerback Trumaine Johnson suffered a hamstring injury in practice on Sunday and may miss the start of the regular season. Now, the Jets will be without another starter for the entire season, something that will test their roster.