Cooper wasn't available after the game and was nowhere to be found Monday in the locker room at the Raiders' facility in Alameda.
Free safety Stuart Schweigert, who was held out of the game after injuring his calf in practice and falling ill over the weekend, spoke with Cooper and said the veteran defensive back had suffered a torn anterior cruciate ligament.
"He's out," Schweigert said. "He's getting an MRI done to make sure the cartilage and all the ligaments are in place. His season is pretty much over with, which is tough because that's the same knee that he hurt a couple of years back. It's disappointing. It's just one of those deals where we'll have to have some guys step up and fill positions, which we do."