In a couple years he could be a key rotational player, and that's just fine for the 23rd pick in the draft.
It was a smart pick by Ainge. The Celtics didn't have the roster spot and wanted to maintain cap space, so they needed stashable guys. Zizic is super young, a hard worker, good motor. Roll the dice and see what he becomes.
One concern I have is that the 7-footer who can't shoot from deep and handle the ball is falling by the wayside in the modern NBA. But then I recall guys like Steven Adams (already mentioned), Bogut, etc. You still need guys like that on your roster to win a title.