I don't see why Tatum can't start at PF by the end of the year. He can shoot and space the floor, pass quite well, rebound better than his size it looks like, is strong/solid, and has the ability to play good defense with some teaching and reps.
He's an elite prospect- I don't know what he won't be able to do by end of season that Morris can, and I don't see Baynes, with his lack of spacing / passing, as a starter (I see him as a bench big that allows us to still have 1 guy with real size when Al sits).