I think the issue with his crossing is threefold;
- Ronaldo plays from that side, so he cannot commit himself forward as much as he did with Mandzukic
- We lack a quality midfielder such as Pogba to hold and distribute the ball quickly from that LCM area into LW space, although Rabiot has improved that recently
- He doesn't have very good acceleration to create a yard to cross the ball, and this has been highlighted even more so with his tiredness
Often when he gets the ball it's in a static position with a right back and supporting defender ready to block, so he looks for the ball back inside. Some left backs in this area will still attempt a cross from this position, thinking that the worst case scenario will be a corner, but he plays a lot more measured than that, especially since Ronaldo arrived. Sarri is not a coach that wants the ball just being fired into the box, a cross has to have purpose.