I did the same thing to my left leg once. I planted it as I was place kicking in football practive just messing around. Well, since I'm not a kicker and shouldnt have been kicking, I hyperextended the crap out of my knee when I went to plant it. Hurt something wicked
It hurts like hell for about 20 min but its walkable and you can play through the pain (as Bennet has shown). In my case it didnt get any worse the next day, but I dont think it got any better either. He should be ok for the 49ers. Im sure of it.