The Celtics big man team is horrible. KG is the only above average player in his role on the entire team. Bass is well below average as starting PF's, Sullinger is well below average as a backup PF (at this point, he will get better), Wilcox is probably about average as a backup center (though he will get hurt any day now), Darko is below average as a 5th big man (and one who has only played in 1 game), and the C's don't have a 6th big man.
Barbosa, Terry, and Green are great at the 1/2/3 and would start on a lot of teams, but you can't just ignore the 4/5 positions which just aren't very good.