Having already qualified for the play-off stages of the competition by finishing in one of the top 16 positions, United were dealt a very early blow as the Saints took the lead through Samuel Amo-Ameyaw.
However, Ethan Wheatley, fresh from his first-team debut against Sheffield United last week, then scored twice inside two minutes as our young Reds flipped the scoreline on its head.
Luke Pearce notched an equaliser for Southampton after 59 minutes, before Sam Mather, Ethan Williams (two) and Charlie McNeill all scored inside a 25-minute period.
Will Merry did grab a late consolation for Adam Asghar's side but it was the Reds who ran out deserved winners.