Form and injuries aside, watching each game last season didn't you get the impression Allegri didn't like Vidal? I think this whole him asking to leave BS is just that. Allegri stated he wanted a true trequartista when meeting with management and also stated that he felt Vidal didn't have a place on his team based on what his vision of the team would be. I felt Vidal was becoming frustrated with being subbed out last season and he only really got major playing time when Pogba was injured. So both parties felt it was best to move on.
