I have a theory about quarterbacks. Assuming a team has a couple of better than average receivers, I think even an average NFL quarterback can become a great quarterback — if he has a great offensive line protecting him.
Kurt Warner was superb for the Arizona Cardinals in the 2008 and 2009 seasons, and to be fair he is definitely a great quarterback in his own right. However, he would not have been nearly as impressive had the offensive line not been as strong as it was those two years.