Benches can only be so good for so long. You are only going to get young players (rookies, 2nd and 3rd year players) or veterans who are on the downside of their careers to comprise a team's bench. When a player does step up, like Posey, one of the Celtics' top bench players last year, then money becomes an issue. So one, a top tier player is not going to want 15-20 minutes coming off the bench and two, a bench player who shows that maybe he deserves more than 15-20 minutes is either going to want to play more or get paid more.
Fortunately for the Celtics, they have players on the bench who are accustomed to a role of being a bench player. They don't demand the ball and will play within a team concept because, for the most part, I think they're happy to be getting paid to play basketball. It's a long season and the Celtics have only lost one out of their first 7 or 8 games. The success of the Celtics still falls on the starting 5 and how well the bench can keep games close while the starters rest. Things aren't that much different from last year and those that remain from last year's bench have a championship year under their belts....so not too much to worry about.