#3 Alexis Sanchez
The Chilean was already an established player when he made the move to United from Arsenal. He had scored 80 goals in 166 matches for the Gunners in four seasons and was one of the better performing attacking players in the league.
Prior to that too, Sanchez had averaged 15 goals a season for FC Barcelona in his three seasons with them. However, since making a move to United, Sanchez has featured in 29 matches and scored only 4 goals.
Mere stats do not tell the whole story here actually. The fact that Sanchez almost never starts for United and actually plays second fiddle to youngsters like Marcus Rashford and Jesse Lingaard is even more disheartening. Mourinho actually played Lingaard in the right-wing against City, when it was obvious that the English youngster was struggling in that position. Sanchez was introduced very late into the match and could do little.
This is no way to treat a footballer, who has been among the best in the world in his position and played in 577 matches in his club career. To make matters worse, Sanchez might actually leave United in January and should that happen, it would underline yet another failure in man-management for Mourinho at United.