FLORHAM PARK, N.J. — New York Jets running back Braelon Allen, placed on injured reserve Thursday with a knee injury, will be sidelined eight to 12 weeks, coach Aaron Glenn said Friday.

Allen, hurt while returning a kickoff on Monday night, is deciding whether to have surgery on what is believed to be an MCL injury.

“There’s two different ways he can go about it,” Glenn said. “He could rehab this … or he could have surgery and get it cleaned up. But the rehab time will be exactly the same. That will be a decision him and his agent will have to make. I’m not making that decision.”

Earlier in the week, Glenn called it “a pretty serious knee injury,” but he withheld a timetable as Allen sought a second opinion. Now they have clarity. The best-case scenario is an early-December return; he also could miss the remainder of the season.

This is a key loss for the winless Jets (0-4), who rely heavily on the ground game. They’re the No. 3 rushing attack in the NFL.

Allen, who has 76 yards on 18 rushes, is their best power back, usually handling short-yardage and goal-line. His absence could mean an uptick in touches for starter Breece Hall, who has a team-high 238 yards on 52 carries.

Isaiah Davis, usually the third-down back, will replace Allen as the No. 2 running back. Davis (five carries for 27 yards) isn’t as powerful as Allen, but he has better speed and cutback ability.

It’s unclear who will dress as the No. 3 running back on Sunday against the Dallas Cowboys at MetLife Stadium. Normally, that role would go to Kene Nwangwu, but he’s battling a hamstring injury and his status is uncertain. He will be a game-time decision, according to Glenn.

On Thursday, the Jets signed veteran Khalil Herbert off the Seattle Seahawks‘ practice squad. Herbert, a former Chicago Bears draft pick, has rushed for 1,905 yards over four seasons. He also can return kicks.

The Jets also made a practice-squad addition, signing former Atlanta Falcons draft pick Avery Williams on Friday. He, too, has return ability.