The 20-year old French midfielder was disappointed at not getting too many opportunities in the United first team during his time at Old Trafford and has admitted to facing Ferguson about his situation.
"I never disrespected Sir Alex Ferguson, if anything it is the opposite, because I said things to his face," he told France Football.
"When I knew of Paul Scholes' return I understood that I would find it difficult to play and that I had to leave United.
"Ferguson told me that in Italy there is a lot of racism and that the fans were not like the Premier League's. I told him that there was no problem for me so I ended up on the team's fringes.
"My team-mates gave me the nickname of Nelson Mandela because I was opposed to Ferguson. But I would say no to (Barack) Obama too.
"(Juventus boss Antonio) Conte, instead, told me that age is not important and that the best players would play. I want to become the best in the world and win the Ballon d'Or."