It's funny... because he didn't make much of an impact during the playoffs, I forget sometimes how successful TA was during the season at filling holes when needed. He never really shone, per se, but he did what was needed to keep things moving toward the victory. Yes, there was the unfortunate foul on Billups in Boston-Detroit I, but beyond that he was a solid defensively and non-toxic offensively. The points that stick out most in my mind are the win in LA when he started at PG for Rondo, the win in Houston when he started for Ray Allen, and the night before that when he started for Ray against San Antonio. All big games on big stages. He didn't take over, or even stand out, but he started against good teams and played good basketball. He's not Posey, and will bring something different than Posey did. But as a wing player of the bench, I think he is capable of doing what is really needed: play tough defense, keep mistakes to a minimum, and be enough of an offensive threat to keep your man honest.
I think it's worth keeping in mind that if our agreed upon goal is another championship, then the goal of the regular season is to prepare for the playoffs by meshing the pieces and winning enough games to get a good playoff position. Even if the loss of Posey costs the C's 10 regular season wins (and I'm not sure it does. They'll win fewer games probably, but more because Doc won't ride the starters as hard out of the gate as he did last year), I still think that a 56 win Celtics team gets decent playoff seeding. So that means they have the better part of the season to work out the first wing off the bench issue, even if that means eventually having to bring someoen else in.