Varejao would be a fantastic fit on this current C's team. He is a rough and tumble player, much like Perk. He wouldn't have much value on the team going into a rebuilding phase though, as he's approaching 30 years-old.
However, the Cavs would only trade him this season for picks or young, promising players like JaJuan Johnson. They don't need O'Neal or his expiring cap space.
The Celts desperately need to keep their picks (for now), but if Ainge could swing JaJuan + filler for Varejao in order to be better right now, then I'd say 'Do it!'