An MRI revealed Thibodeaux suffered a sprained MCL, but his ACL was intact, NFL Network Insider Ian Rapoport reported, per a source informed of the injury.
Rapoport added that the rookie's timeline for recovery is about three weeks, which keeps the door open for a Week 1 or Week 2 debut.
Confirming Thibodeaux's diagnosis on Monday, Giants coach Brian Daboll considered the rookie pass rusher as "day to day" but wouldn't forecast a return.
"Fingers crossed," Daboll said. "Hopefully his rehab goes well."
Thibodeaux's diagnosis confirms the optimism from New York after sustaining a scary, but legal, low hit early in the preseason contest.
Thibodeaux's right knee bent inward on a block by Bengals tight end Thaddeus Moss. The 21-year-old went to the turf in pain and was attended to by trainers. The rookie walked off the field under his own power and was taken to the medical tent before finally heading to the locker room.
Before exiting due to injury, Thibodeaux started and played 15 snaps, logging one tackle, in what will be his final preseason action as a rookie.
Daboll also announced on Monday sixth-round rookie linebacker Darrian Beavers suffered a torn ACL during Saturday's game and will be out for the 2022 season.
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