One thing in Leinart's favor is that he's been leading a national championship team for so long and doing it with turnover at key positions. It hasn't been Bush carrying the team the past two year's and Leinart getting the credit. This year, I think Leinart's been unspectacular to say the least.
I wasn't aware of his shoulder issue so that is a good theory. It seems that most of the USC passing game is short to intermediate stuff but I don't know if that's by design or because Leinart's shoulder is bother him. I don't catch all the games but the handful I've watched this year I don't see him slinging the ball downfield with any kind of authority. For the most part, he still seems to be making good decisions which will work in his favor.
Actually, as good as the two wr's are for USC this year I don't think they're of the same caliber as in previous year's, so Leinart may be limited in what looks he's getting downfield. USC seems to stay in pro set's most of the time, no 4 and 5 receiver, run & shoot looks for the most part, so it's possible that he may be having to dump the ball off to the back's rather than force the ball downfield if there's no seperation. If that's the case, then he's learning exactly what he'll need in the NFL and his stock should stay strong. If he's just unable to get the ball to open receivers and his shoulder, knee and concussions become more of an issue then he might struggle or fail in the NFL.
All in all, I think he can be a good NFL qb if his smarts and leadership are real (rather than manufactured by the media). He has the size and strength to make it but he will need to be taken by a team that will cater to his strengths and not depend on him alone to win games. Houston drafted the face of their franchise and he's been getting killed. After all these years, the Texans still don't have a good line. Now, Carr isn't the untouchable icon that he once was. But, if the Texans lay the blame on him, they'll be falling into the same trap that the Cardinals have.