I think Mourinho is not the manager best suited to getting the best out of him. I believe he'll do better at Arsenal.
Saying that I also don't think Sanchez is either, it is an odd decision for him to choose United. Obviously they offered him more money and I can't blame him for that but I don't think he'll be as successful there as he'd have been at City.
I agree with you about Alexis Sanchez, not suitable for United same as Mkhitaryan. But anyway I want to see him play first and then I will comment his performance, so let's see what he can do. About Mourinho well I disagree, ok maybe he talks too much, but I think he has managerial skills for sure. United after Sir Alex had terrible bosses, that destroyed the team and now someone must rebuild it, I think Mourinho can achieve that!
I agree with you. His major motivation was money. If not, it's easy to go to City and secure the League with them rather than moving to a United side that don't really yet have what it takes to secure top spot.
But we can't blame a man for making his decision. Various things serves as motivating factors for us as human.