On what basis do we have the right to assume it won't? 
Uhh...gee maybe because he hasn't played in a year he's had no training camp to get into basketball shape and he's never been forced to play the kind of defense he's gonna have to to be effective at the defensive end for this team. Sam Cassell had a ton of issues figuring out the offense. By Steph's own admission he wasn't prepared nor in game shape. On top of all that he still has to learn the plays and sets. And again this is a guy who has NEVER come off the bench and is not used to be just a role player.
I'm not saying it's not possible but to just assume that everything's gonna be alright, "no sweat" and couple weeks Steph's an all-star coming off the bench. That just doesn't seem rooted in reality. I'd like to think it's that simple but it rarely ever is.
I agree that he may not shake the rust. However, I keep in mind that he has averaged 19 ppg, 7.8 apg, 1.2 spg over his career (including his time in Boston.). Now he may never come close to these numbers in Boston and he certainly will not become an "all-star" by the end of the season, but we don't need an "all-star" off the bench.
If things go right, the C's will be playing into June, and by then I feel that it is safe to assume Marbury will be in fine basketball shape considering that he isn't horribly out of shape or overweight right now, and I also think it's safe to assume that he will be able to get us 5-10 ppg, 3-6 apg, .5-1 spg, which is all we really need from him.
I really have no idea what will happen, but I am hoping that he can contribute in some capacity, and it will be fun to watch and see how he develops with the team.