It's a good bet that the Mavs will keep the flexibility and leave those spots open. The reality is the 6-foot-10 Thomas was a nice insurance policy, maybe a spot player here and there that could drain a 3-pointer, but he was never going to crack the top nine currently on the roster.
It is true that with no Thomas, the Mavs are down to one true power forward in Dirk Nowitzki. They'll rely on 7-foot-1 center Tyson Chandler and 6-foot-7 small forward Shawn Marion to man the 4 when Dirk's out.
Marion played some 4 last season when the Mavs rolled with a small lineup and the Mavs feel Chandler is versatile enough to get the job done.