I think Ainge can dump Bass and not lose much because they have a good replacement in Humphries to take over those vacant minutes. Trade Humphries as well as Bass and Boston would have a serious shortage of reliable big men.
Trading G-Wallace would be a bigger loss for the team because he is the only other SF with legit size on the team after Jeff Green. Keith Bogans would take his minutes, a lesser replacement.
Courtney Lee would be a sizable loss in the here and now but after Rondo returns, given the way Jordan Crawford is playing, they could probably navigate things without C-Lee.
So a large downgrade at the PF + C positions and add to that the downgrade at SG + SF and Boston would go from having one of the better benches in the league to having one of the weakest. That is probably 10-12 wins right there.
PG: Rondo, Pressey
SG: Bradley, J.Crawford, M.Brooks
SF: J.Green, Bogans
PF: Sully, (FA)
C: Olynyk, Faverani
Stevens would still have the first choice starting lineup in place for the most part (Sully instead of Bass, a change I would like). So the team wouldn't be rubbish. But the downgrade to the bench would be costly (10-12 wins I guess).