Romelu Lukaku strengthened their grip on the encounter by converting from among a group of players in the box from a Daley Blind cross following a short corner.
Christian Benteke's equally stunning overhead kick had thrown Liverpool a lifeline after Daley Blind's crisp finish and Ander Herrera's penalty had put Louis van Gaal's side in control.
United deserved the victory - enjoying 63% of possession and having a total of 25 shots on goal - but only a first-half Daley Blind effort separated the teams until the final 10 minutes.
Neither side managed to impress in the shootout, but Man United edge it in the end as Casemiro hit the bar after Blind managed to give the Red Devils the lead, though the Brazilian is hardly the villain for Los Blancos who netted just one of