His main asset was that he was a balanced midfielder. He had the stamina to move up the field and play the B2B role next to Xabi Alonso. He could provide cover for Ozil's lack of defending with his positioning. He wasn't a flashy player in Madrid but he brought some balane to the side.
But when Madrid moved to a more classic 433 he lost his place in the side because he needed to be more dominant on the field. We dominate games much more and expect him to playmake now and Khedira isn't exactly a playmaker. For the tireless running we have Matuidi who does that better. We need someone with more playmaking capabilities
We should probably acknowledge the fact that he shouldn't start anymore in Serie A games. We could use him in the CL but for Serie A I'd say Bentancur should play over him.