The two squads contested a behind-closed-doors friendly at Cobham, with a strong United side containing first-team squad members Gary Neville, Danny Welbeck and Ben Amos.
Club captain Neville completed an hour, while Welbeck put United into an eighth minute lead as the Reds started strongly. However, a quickfire double from Franco Barini to put the hosts ahead just after the half-hour mark.
The Blues struck two further goals, late in the second half, to secure a 4-1 victory - inflicting United's second defeat in seven pre-season outings.
The Reds' next action comes next weekend, taking on Shamrock Rovers' first team and Arsenal's Reserves in the Platinum One Challenge.