I really think that its due to injuries and being in a bad team. A fullback can't look great when he is surrounded by garbage. He looked good when he played in a good Milan and Italy team. The same will happen here. In the end he is a starter for Allegri, Conte, and all the coaches Milan have had when he is fit.
I think he will end up being a huge bargain. Not many people were optimistic about Pirlo and Barzagli. I am hoping he succeeds