He would not be a long term foundational starter replacement for Horford. He would be a valuable rotation big to fill in for missed games and cover the spots where Grant Williams isn't big enough. For that he is pretty ideal.
Absolutely, no way he is starter material. In my mind, he would slot somewhere between Grant and Kornet in terms of the big rotation or depth chart. I am not nearly as excited about him as some seem. I see him in the Hauser, Pritchard range in terms of depth or impact:
Starters: Smart, Brown, Tatum, Horford, RWIII
Rotation: White, Brogdon, Grant
Bench: Hauser, Pritchard, Olynyk(?)
End of Bench: Kornet, Griffin
Towel Waver: Jackson