Rivera was injured in the first quarter of Green Bay's 20-7 loss to Carolina Saturday night at Lambeau Field. Panthers defensive tackle Kris Jenkins rolled up on Rivera's knee from behind during the Packers' second possession. Safety Darren Sharper was also injured on that drive, which started from the 1 yard line after Green Bay's goal line stand kept Carolina off the board.
Rivera was down for several minutes before being helped from the field. Things didn't look good when Rivera was taken to the locker room on a cart.
The preliminary diagnosis was a sprained medial collateral ligament.
"When I talked to the doctors, there is a chance he could be ready for the opener, believe it or not," Sherman said in his post-game press conference. "But there's a chance he won't be, either.
Rivera earned Pro Bowl honors last season after playing with tears in both knees. He underwent surgery in the offseason.
If Rivera is unable to play vs. Minnesota Sept. 7, it will mean that the Packers have not had their starting offensive line intact since Week 2 of last season when Mark Tauscher suffered a knee injury in New Orleans (Sept. 15).