The attitude of the players was not professional
The United players went to Bristol City expecting to comfortably dispatch them and move on into the semi-finals. This was far from the case.
Their Championship opponents were in their faces for the entire 90 minutes and consequently rewarded with a famous win in-front of their boisterous supporters.
Top quality players including Anthony Martial and Marcus Rashford did not perform to the standard that is expected of them usually, which could have been due to complacency.
The players should have approached this cup game as they do any other match in the Premier League or UEFA Champions League - they failed to, which has ultimately cost them.
The fact that Mourinho himself publicly revealed post-match that perhaps players did not want to turn up to play is a very worrying sign, a lack of professionalism.