Tommy will definitely be in the mix and, despite SoCal's endorsement, will definitely be the favorite to win the position. While he has significant areas to improve, his experience edge alone IMO, is nearly insurmountable for the first 2015 game. I do think Riley will open up the competition, and others will get a good look. We will know quickly who the contenders are but it is likely there will only be one (Stanton, but possibly Darlington). If we see that QB getting some significant rep's in the first or second game...then we will know that he is being prepped to replace Tommy, if TA at some point falters.
While TA's game against USC could have been a disaster had USC intercepted just a few passes that seemed destined for interception...all in all, he ended up with a pretty good game statistically. Keep in mind though, that good passing days against USC this past season has been more the norm than the exception. Believe it or not, teams averaged over 275 yards per game and about 60% completions against them.