Didn't Philip Rivers have a partially torn ACL in 2007/2008 and plays with it in the AFC championship?
Then he came back fine the next season.
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So? 14 to 18 months is a butt load of time and those injuries can change your mentality of the game. RGIII is young... He'll have to come back with the mentality that he has to play no matter what. If he comes back and starts worrying about taking hits because he thinks he'll ruin his life with another injury, then he's screwed.
It's perspective, but it does happen to people. Especially since he's a mobile QB too...