Dylan Harper's thumb injury pretty much erases any possible confusion about who is going to start between him and Stephon Castle when the regular season gets going. The Spurs already announced that the Rutgers rookie would miss most, if not all, of training camp. He may even miss time when the games start, so there's no question that Castle will be in the opening unit.
He was already projected to get that spot, but those are just educated guesses. Who knows what would have happened once training camp began? I believe that Mitch Johnson would be willing to start whoever earned the role.