#1 Managerial tactics
Last time against Manchester United, Jurgen Klopp went with the 4-3-3 formation. His midfielders were Fabinho, Naby Keita, and Gini Wijnaldum. I expect him to go with the same formation and same midfield.
But Fabinho was brilliant at the centre back position against Bayern Munich, so Klopp could play Jordon Henderson in the holding role instead and Fabinho with Virgil van Dijk at centre back. Also, Wijnaldum could be tasked with man-marking Paul Pogba, who is the clear danger man from Manchester United.
Ole Gunnar Solskjaer has usually played in a 4-4-2 diamond against big opposition and I think he would go with the same tactic. If Jesse Lingard and Anthony Martial are fit, it could be a repeat of the system that Solskjaer deployed against Tottenham Hotspur. But if they aren't, then Lukaku and Rashford could be playing the roles of wide forwards and Juan Mata could be playing at the centre of the diamond.
Jurgen Klopp clearly has more managerial experience than Ole Gunnar Solskjaer, and that could prove to be decisive.