If he were good enough, he would have seen the open receivers, and there were all too many occasions where he didn't even look for Styles or other receivers, right as they were breaking wide open. He was so locked into Mayer, that he was all but blind to the others.
Right away, that would have made the difference in the Stanford game. While there was plenty of blame to go around (not kicking the field goal, blown defensive assignments, Estime's fumble), the ability of a QB to read multiple receivers and go through the progression, instead of locking onto one receiver, would have easily made this game a blowout in our favor.
Maybe that flaw comes from the lack of adequate coaching by Rees, since he was essentially spoiled by having two 5th year veterans during the last two years (Book and Coan) who could go through those progressions on their own.