Bradley will start for a few reasons:
1. Bradley is the better player and the player with the most Celtic tenure. Those type of players don't lose their starting position due to injury unless the player that replaces them shows clearly that he is playing better than the vet or that his presence severely effects the way the starting unit performs as compared to when the vet played. Ray never got his position back from Bradley because Bradley's insertion into the starting lineup completely changed the way the team played defense. There was a huge difference between the way those starters played defense when Bradley was in the lineup as compared to Ray Allen. Thats not happening here. Brown's presence isn't making the starters better and he isn't playing better than Bradley.
2. Bradley sets a defensive tone straight from the start. He hawks the ball handler and pressures the ball. Stevens has mentioned numerous times that Bradley was tremendoysly important for setting a defensive attitude from the start of the game. Brown is a lot of things but a defensive difference maker isn't one of them. Bradley is such a player.
3. If this team had a captain it would be Bradley. He is one of the most respected players on this team. The players, probably even Brown, feel he should be the starter. Giving his position in the starting lineup simply to hide Brown's deficiencies when he plays and maximize a rookie's development is a good way for a coach to lose respect in the locker room. You don't screw with a defacto captain's starting spit for the sake of forcing starters minutes to a rookie. Good coaches know this.
4. The bench's scoring problems can be solved be staggering the substitution patterns to have Beadley play with the bench more. Its what Stevens was doing before Bradley got hurt. There is simply no reason to bench Bradley so that he can play with the bench. No reason whatsoever.
Btw, Eddie20, the reason McHale and Hondo came off the bench is because there were All-Star level veterans playing in front of them and the Celtic philosophy was for young players to come off the bench, sometimes for years, until the veteran in front of them were no longer on the team. Once those vets were gone, then Havlicek and McHale took their place in the starting lineup