Both played out a terrible first half, with very little quality on display.
But there was an immediate change in proceedings minutes into the second half.
Juan Mata's well disguised free-kick from the left picked out an unmarked Daley Blind on the edge of the box and the Dutchman calmly side footed a left footed shot into the top corner with Simon Mignolet rooted to the spot.
Blind then made the difference at the other end with two goal line blocks to deny Danny Ings and then Roberto Firmino.
New £36million signing Anthony Martial came for his debut in the second half.
A reckless challenge by Joe Gomez on Ander Herrera in the area gave United the chance to double their lead and the Spaniard made no mistake with the penalty.
Liverpool got themselves back into the match with a sensational overhead kick by Christian Benteke with five minutes to go.
But Martial upstaged Benteke with a glorious goal of his own.
The Frenchman beat three men before finishing with a goal reminiscent of Thierry Henry.