I'm not sure that I'm comfortable with the backup SF situation, HOWEVER for those that are saying TA is not a proven rotation player - he played 18 minutes a game last year for the world champs. While he is not the ideal choice to slot into the SF position as a backup, he is capable of making regular season contributions to the team as a rotational player. I'm on board with the question of whether his size will be a detriment at the job, but let's not dismiss the man as a useless scrub when he has already shown he can be a rotation player.
That said, I think that for the 15 minutes a game that Pierce is not in (which will usually be a time when they are facing the opponent's second string anyway) during the regular season, the Celtics will be more than able to survive with what they have. Again, not every team has an unstoppable SF who will be out on the floor the whole time Pierce is resting. The goal for the second unit is to maintain and possibly extend the lead established by the starters, not shut down the other team's bench and go on a 15-0 run. With the personnel they have, that is an achievable goal (the maintaining thing, not the 15-0 run part). In the playoffs (and against key regular season opponents), Pierce will be playing substantially more minutes, so the backup question is reduced to someone to fill the void long enough for him to get a breather.
Remember, the goal is to make the playoffs with home court without exhausting the starters, at which point the rotation gets cut back and the starters play the lion's share of the minutes. Will they need 66 wins again to get home court? Probably not. So they can give up maybe 6-8 wins and still be ready for the playoff run.