#2 Angel Di Maria
Di Maria was at Manchester United for just one season and his departure was due to his fallout with Louis van Gaal in the former's very first season. Di Maria is a world class player and everybody knows that, but I just think that Van Gaal could have handled the situation better at that time as you really don't want one of your better players to leave.
But that didn't happen and since his switch to Paris Saint-Germain, the Argentine has been showing just what he is capable of in France. With United lacking a genuine right-winger and a playmaker, Solskjaer would have loved to have him at Old Trafford.
Even Mourinho admitted in his first season at the club that he would have never sold someone like a Di Maria but it is what it is.
#1 Cristiano Ronaldo
This is the transfer which most Manchester United fans talk about and there are two reasons for it, one - Ronaldo was sold when he was the best player in the world, and second - Manchester United never replaced him.
The second reason is what has defined United in the last couple of years. The Red Devils have not been able to replace their best players and since David Beckham and Ronaldo, United haven't had a genuine right winger which just tells you the lack of structure at the club.
I don't blame Sir Alex Ferguson for the Portuguese's departure but I think that the United board could have done better to keep their best player. But that didn't happen and now Manchester United are in dire need to add some characters such as Ronaldo.