We didn't. He was the one who left. Fergie even sent Evra to persuade him to stay.I wish we had never left him go originally. As for his present troubles, the team system doesn't suit him. I'd bet Sir Alex would have been able to slot the Paul Pogba of now into the United team with max efficiency.
He's world class and exciting but psychologically playing for United under Mourinho seems to have done some damage.