The Celts don't really need any additional big men. They've got plenty.
So for mid-season buyouts, I'd look at wings, or possibly veteran ballhandlers if the team suffers injuries to the young guards, or Smart and Rozier seriously struggle to run the offense without Thomas on the floor.
Perhaps Lou Williams could be a name that comes up. Would the Lakers trade him for, say, Tyler Zeller and a pick?
If Rudy Gay actually does get bought out, I could see him really thriving as a second unit scorer. He could play strictly at the three and just do his iso-ball thing, and the guys around him could focus on playing defense and getting out on the break.
Would probably mean Brown spends the second half of the season on the bench, though.