Steven Gerrard's first-half penalty was extinguished by a classy Wayne Rooney equaliser before Juan Mata's defelcted effort put United ahead a minute later. Substitute Jesse Lingard ended the contest late on.
Here are five talking points from the Sun Life Stadium...
Liverpool became the first side to aggressively push high up against United in pre-season and it showed. Jonny Evans, the best footballer of United's back three, suddenly looked uncomfortable, Chris Smalling's distribution is no better than it was when he joined United four years ago and it was inevitable Phil Jones would make a vital error as he conceded a second penalty to Liverpool this year.
United have had issues with pressing against sides and handling opponents' energy as far back as the 2009 Champions League final. They have shown progress under Van Gaal, however Liverpool's determination to regain the ball was a reminder of how much more intense it will be dealing with Premier League sides.
LAMBERT COULD BE CROUCH MK.II
Steven Gerrard robbed Juan Mata of the ball, drove forward, played the ball wide and Luis Suárez made it 2-0. That might have been the outcome if the Uruguayan was still a Liverpool player (and hadn't bitten Giorgio Chiellini), but instead it was Rickie Lambert who rolled a backpass into David de Gea's path.
Lambert will not be first choice at Liverpool but his barren run - and Daniel Sturridge's injury - has re-emphasised Brendan Rodgers' need for another striker. Lambert is more skilful than he is often given credit for, but he was the one attacker who looked off the pace in Liverpool's dominant first-half and although it is only pre-season there are already symptoms of Crouchitis.
YOUNG HAS SAVED HIS UNITED CAREER - FOR NOW
Ashley Young is fortunate to still be at United after three dire years in which he peaked four games into his Reds career, however no one has benefitted more from Van Gaal's arrival than he has.
One of the most unlikeable players amongst supporters for his diving and attacking bluntness, Young was excellent again on Monday night, just 48 hours after excelling against Real Madrid. Forced to switch flanks following Antonio Valencia's eighth-minute withdrawal, his crossing was reminiscent of his carefree Aston Villa days and his versatility on both wings will please Van Gaal.
Young has always worked tirelessly in Red but failed to supplement it with incisiveness. That has not been the case in the United States.
COUTINHO COULD BE SUÁREZ'S HEIR
It is a damning indictment of Luiz Felipe Scolari's management he omitted Philippe Coutinho from his Brazil World Cup squad when donkeys like Fred and Jo were preferred in attack. Coutinho was Liverpool's outstanding performer at the Sun Life Stadium and there are more encouraging signs for Rodgers that he and Raheem Sterling could form a devastating triumvirate with Daniel Sturridge.
Coutinho's speed of thought had United in a spin in the first period before Liverpool became complacent after the pause. Preying on United's nervy back three, his touch and movement was unrivalled during an ebullient 45 minutes that reinforced his importance to the Reds. Adam Lallana is viewed as a competitor but on this evidence Rodgers will be spared a selection headache.
SHAW GETS INTO THE GROOVE
"Lucky" to play against Internazionale, Luke Shaw was the early replacement for Valencia in Miami and ended the game as United's best player. Van Gaal's criticism of his fitness was justified but Shaw's energy levels in the Floridian humidity were matched perhaps only by Young on the other wing.
Helpfully for Shaw, Liverpool focussed their attacking on Raheem Sterling down the left as Coutinho drifted across the forward line. That allowed Shaw to maraud forward regularly and develop his telepathic understanding with Wayne Rooney and Juan Mata, which was evident with some neat one-touch link-up play.