I've always liked Belinelli. I still think he is more of a big PG than a small SG and a much better player at the point too.
If Boston plays Jason Terry against Belinelli, he has that PG type matchup. The size advantage to see (pass) and shoot over the top of the smaller defender. Terry is gifted enough offensively to make up for that on the other end but it'll probably be a neutral matchup. Maybe even an advantage for Chicago if Terry continues to play a peripheral role offensively.
If Doc goes with Courtney Lee, Boston has an effective defensive option on Marco but the two will neutralize one another. Maybe less if Lee continues to misfire with his jump-shot.
Marco Belinelli struggles with size defensively (and offensively for that matter). Even with solidly built two guards. Nevermind a power three like Jeff Green or Paul Pierce. Those two guys could really do a lot of damage against Belinelli. That is where Boston's most advantageous matchup is against Belinelli.