The part that I wonder (and I don't know anything about Cooper's injury) is if he had a knee sprain that is bad enough to miss at least 1 game, why was he allowed to re-enter the game against Denver possibly injuring the knee more?
Because it is hard to gauge those kind of things during the game. If you're playing, you are loose and limber, your knee hasn't swelled or affected your range of motion, you have adrenaline to temper any pain. Once the game ends and you rest your knee, it swells and gets stiff and hurting a bit more than it did during the game.
It is very much a good sign that he was able to continue playing last game, anything serious would have affected him immediately. Minor sprains can be played through initially but you risk aggravating it further if you come back too soon. My take after having multiple knee injuries myself.