Winners of the competition for the first time in 2024, United were defeated on penalties in the quarter-finals last season by eventual victors, West Ham.
Darren Fletcher’s young Reds had begun the league season in good form, defeating Everton 1-0 away before a 5-0 thrashing of Middlesbrough at Carrington last Saturday.
Back at home under rainy skies on Wednesday lunchtime, Fletcher’s boys went into the break leading after a dominant first half without many clear-cut chances, thanks to Godwill Kukonki’s powerful shot from a corner.
With Brighton keen to turn things around after half-time, United capitalised on spaces opening up and scored three goals in quick succession through captain Amir Ibragimov and then wingers Bendito Mantato and Samuel Lusale.