United's Under-18s missed the chance to close the gap on league leaders and local rivals Manchester City after slipping to a late 1-0 defeat on Saturday.
Ahmed Benali scored the only goal of the game at Platt Lane six minutes from time, ending the Reds youngsters' unbeaten away record this season and extending City's lead at the top over Paul McGuinness' charges to nine points.
McGuinness handed a first competitive start to striker Josh King as United, who lost this fixture 5-0 last season and were beaten 2-0 by City at Carrington in December, gave as good as they got for most of the match against an older, more physical City outfit.
Greg Cunningham was forced into a goalline clearance to prevent the Reds taking a first-half lead, before the same player sliced wide when well placed in the United area.
City were the better team in the second half and their breakthrough came when Benali controlled a cross from the left, escaped Joe Dudgeon and beat Conor Devlin's dive.
Captain Oliver Norwood came close to rescuing a last-gasp point for United, who had struggled to create many clear-cut chances, but the midfielder's fine volley was superbly tipped away by City goalkeeper Joe Clegg.
United: Devlin, Tunnicliffe, Dudgeon, Thorpe, Wootton, Norwood, Ajose, Morrison, King (Macheda 73), Petrucci, Brady (Curran 77).
Subs not used: Gill, Woods, James.