To me, Lee is an upgrade over Bass and he can play center. This trade also makes us longer by moving Turner to SG and getting Barnes at SF. It takes Thornton and Bass out of the lineup, freeing up minutes to give to our younger SGs and PFs.
It also lines up our big contracts to come off at the same time: Green, Lee, Wallace ($35 million). If in 2 years, they all walk and we get nothing in return: we still have a roster loaded with younger players, cap space, and Young in his 3rd year would be looking for a breakout year with Green gone.
In the meantime we have a solid first team: Rondo, Bradley, Green, Olynyk, Lee and a pretty dang good second team: Smart, Turner, Barnes, Sullinger, Zeller.