I think we are seeing the effect of having no PG against a very good team playing second half playoff defense (AKA, locking it down).
You can get away with no PG in the regular season, by running, and taking advantage of defenses that aren't really locked in. But, in the playoffs, you need someone to break down defenses and create shots, and the C's just don't have that.
Unfortunately, the C's season ended when Rondo went down.