#3 Unhelpful refereeing decisions
This might sound ungainly, but Manchester United did suffer from a lot of terrible refereeing decisions on Wednesday. Referee Gianluca Rocchi allowed Barcelona's holding midfielder, Sergio Busquets, to play on after he had brought down Scott McTominay with his high challenge early on in the game.
He was booked later for another high challenge on Paul Pogba but escaped a second yellow card after another push on the Frenchman in the 38th minute.
To make matters worse, in the latter stages of the game, there was an appeal by United for a handball by Busquets off a United cross, which was turned down, and the Spaniard escaped lightly again.
If Busquets had been sent off, United would have had a clear advantage in terms of numbers. Also, if the handball against Barca had been given, the Red Devils would have had a penalty.