I think the brace will stabilize the knee when he runs and help him against mild hits. I was on a soccer field about 3 years ago where a goalie and his brace got blown up by a tackle in a collision. It was pretty ugly, the goalie blew out his acl again and his brace was broke and bent. So no, I don't think a brace will help with a serious hit.
I do know the new rule will benefit all QB's knees in the future though. I don't agree with the rule and I don't agree with people calling it the Tom Brady rule. What's wrong with the Carson Brady rule. Or the Pollard/Wilfork rule. joking.