Yeah, Kostic was a LWB already, playing LB should be doable to him. With Pogba and Chiesa infront, he will be bombing in more crosses than now, with space.
If you ask me, get rid of Bonucci and Rugani, then we will need a starting caliber CB regardless of Danilo. But he can really play anywhere in a backline, and we need options to rotate and for injuries so its fine.