Gotta be Yabu, right? Clearly there is the chance the Davis deal goes down and that would mean Tatum's chances would be high.
But, separate from the Davis trade is whether Yabusele warrants another year here. I think it clear to most that it's time to move on from Yabusele. He appeared to get Big Baby/Jared Sullinger disease and somehow put a lot of fat onto his body while playing professional basketball during the season. And his skills are pretty bad.
So he might be in the Davis trade as salary filler, but unlike Tatum, aside from the Davis trade, there is the added chance Yabu is traded because he is salary dumped or moved for a small asset.
Yabu also might need to be moved to make way for a 1st round pick if the Celtics have to make all 3 picks. It's pretty much a given that an unknown commodity mid first rounder holds more value than Yabusele does currently, so it makes sense to move him to open a roster spot to a pick.