"We dodged a bullet there," Buffalo general manager Buddy Nix said. "It's not a season ending injury. He won't need surgery. It'll be about two weeks."
Posluszny was hurt on the first play of the third quarter when he was one of three Bills defenders to bring down running back Ronnie Brown along the sideline. Posluszny sat on the field for several minutes before getting up on his own.
Trainers examined Posluszny's right knee before having him jog off the field. Posluszny returned a few minutes later and ran along the sideline while wearing what appeared to be a brace before he was escorted back to the locker room.
Following practice Monday, Coach Chan Gailey didn't disclose any details of the injury, except to say tests revealed it wasn't as severe as first feared. There's a possibility Posluszny might not be back until Oct. 24, when Buffalo returns from its bye week to play at Baltimore.
The injury is the latest setback for Buffalo's 2007 second-round pick out of Penn State. He missed most of his rookie season with a broken left arm, and then missed four more games last year after breaking a bone in the same arm.
The Bills are already thin at the linebacker position as they prepare to play at Green Bay this weekend.
Starting outside linebacker Reggie Torbor did not play Sunday while continuing to recover from a chest injury. Gailey said there's a chance Torbor could be ready to play this week.
Gailey said reserve safety Cary Harris will not play this weekend after hurting his hamstring against the Dolphins.
The Bills also signed tight end Rob Myers to their practice squad. He replaces tight end DajLeon Farr, who was placed on the practice squad/injured list due to an undisclosed injury.
Listed at 6-foot-4 and 240 pounds, Myers was available after being released by New England on Sept. 4. He split last season on New England and Philadelphia's practice squads. He was initially signed by the New York Jets last year as an undrafted free agent out of Utah State.
ESPN NFL Insider Adam Schefter contributed to this report. Information from The Associated Press was used in this report.