I think Robin Lopez would be great on this Boston team.
Balance out the lineup. Keeps everyone else in roles they are comfortable with and can be successful in ... and I think they is very important for developing young players.
TP. I think this is an under appreciated aspect of rebuilding. I think it would be very helpful to have at least a serviceable center to allow our developing bigs (sully + olynyk) the right lineups to learn and not have to play out of position all the time. I don't feel either will do well with having to get killed every night playing center when it really isn't the right spot for them.