The Reserves suffered a first league defeat since early March against Liverpool, thanks to David Ngog's early strike and a string of missed chances for Ole Gunnar Solskjaer's side.
French striker Ngog struck after defensive hesitancy as Liverpool bossed the opening exchanges, but how United's second-half dominance - against a side reduced to 10 men - failed to produce at least an equaliser is a mystery.
United made a nightmare start to the evening when a long punt forward from goalkeeper David Martin was mis-controlled by Ngog. The Frenchman advanced on goal, shadowed by United's Ritchie de Laet and Joe Dudgeon, but Ngog reached it first to poke a finish past the off-balance Ron-Robert Zieler.
Confidence temporarily dinted, United were on the back foot for the opening exchanges, and required a goal-line clearance by David Gray to keep out Nathan Eccleston's close-range effort.
The Reds did gradually gain a foothold in proceedings, with Kiko Macheda denied after a left-footed snapshot, but it was Eccleston who had the next clear chance as, having worked space well, he powered in a shot which Zieler comfortably beat to safety.
United's chances were aided by the decision to send off Liverpool's Damian Plessis, just after the half-hour. The French midfielder lunged into a challenge with Magnus Eikrem - whose foot was also high - but his offence was deemed worthy of a red card.
The numberical handicap unsurprisingly prompted a mass retreat from the hosts, and United took the initiative by totally dominating matters from then on.
Early in the second half, Josh King twice latched onto Zoran Tosic through-balls, only to see his first effort well saved by Martin, while he was then denied by a last-ditch challenge from Chris Mavinga.
Danny Welbeck missed a clear opening when he failed to connect with Magnus Eikrem's in-swinging free-kick. Liverpool, for their part, should have doubled their advantage when Ngog headed a Jay Spearing set-piece over the bar from close range with 20 minutes remaining.
Macheda was fractionally off-target shortly afterwards, toe-poking just wide after some fancy footwork amid a cluster of five home defenders, and the Italian was denied an assist when James Chester nodded his cross off-target from just inside the six-yard box.