#2 Performance of both midfields
There is no doubt that Manchester United's revolution has taken place due to the fact that Nemanja Matic, Paul Pogba, and Ander Herrera have been in terrific form. Against a Liverpool side who tend to give very little time on the ball, the efficiency of these three could prove to be very important.
Also, the pressing has to be very precise and in order, because this Liverpool side can really hurt you, even if they are given a sniff.
For Liverpool, Gini Wijnaldum and Fabinho have been in terrific form recently and they would like that to continue. The biggest responsibility for Liverpool's midfield on the day would be how to keep Paul Pogba in check. Also, not to allow Nemanja Matic and Ander Herrera control the game's tempo. If Liverpool is able to overrun Manchester United's midfield, then it would spell trouble for Manchester United.