Maybe he was just going through an early mid life crisis? He clearly had his heart set on Manchester and probably took him too long to get over it much like trying to get over a girl that u really like.

He had a bad knee injury which plagued him for months and wasn't adequately treated as he himself, Conte and the Chile coach didn't give him any rest. Now he's slowly regaining form, the United BS the media spewed during summer has nothing to do with it.
