Everyone keeps saying "we gotta trade for a big, we gotta trade for a big".
Let me throw a monkey wrench into the discussion.
I still maintain (and bear in mind I missed the Dallas and San Antonio games) that our lack of ability to score on mid-long range jumpers is what is killing our post and under-the-basket game.
As long as we can't hit the broad side of a barn from outside, teams are free to collapse in toward the middle, cutting off passing lanes (thus no ball movement except on the perimeter) and preventing easy lay-ins or dunks - not to mention blocking anyone who might accidentally be around for an offensive rebound.
This isn't double-teaming, just a lane-centric defense that challenges us to hit our outside shots, which we then oblige by bricking.
I would say that we need a consistent offensive presence in the 1/2/3 position to relieve the pressure on the inside and give our existing front men some breathing room.