#1 A controversial decision, and Salah's absence
It can be argued that a few crucial decisions went against the visitors on a night when they were left frustrated by a resolute United side.
Liverpool fell behind through Rasford's strike in the 36th minute, but during the build-up to that goal it seemed as though Divock Origi was caught by Victor Lindelof. Replays showed that the Swedish international had caught Origi around his calf. At the same time Andy Robertson was caught out of possession, which allowed Daniel James to run down the left and assist Rashford.
This incident could have been significant had VAR intervened and reversed the on-field official's decision. The entire result could have changed had United's goal been disallowed over the foul on Origi.
In addition, Liverpool clearly missed Salah in the game. In spite of having a dismal record against United, Salah could have made his trademark off the ball runs to draw defenders out of position. That could have worked massively in Liverpool's favour as it would have allowed Mane and Firmino some free space to exploit.
Origi did not make enough runs and failed to stretch United's defence, as a result of which he was substituted at the hour mark.