Because quad contusion sounds a hell of a lot better than “you know that arthritic knee that he’s been having trouble with?  Yeah, it’s already a definite issue after five games averaging 13 carries per game.  Not looking good for this season.  Any other questions?”
As I said upthread, if it were a 
quad contusion, I would think video of the hit where it occurred would be circulating by now.  No?  Not saying it didn’t happen and they’re running interference with what they’re saying, but a quick video clip of the hit that caused it would significantly quell concerns.