I think he lacks the true hunger and dedication to make the most out of his career. From watching his recent games it quite striking how casual he plays, strolling around the pitch and not imposing himself enough to make a real impact on the game. He's always been like this tbh but Conte and Allegri got much more out of him than he is showing right now.
He is not worth the fee and salary if he doesn't have a manager that demands hard work or isn't surrounded by players that won't tolerate such behavior. With his talent, if he is satisfied with fighting for a top4 finish, then that's probably all you need to know about his ambitions. He probably has one way out of Manu because no one will pay his transfer fee and that is by not extending his contract.