Brighton striker Glenn Murray scoring past Manchester United goalkeeper David de Gea in the 25th minute to give the home side the early lead in yesterday's Premier League clash.
Barely two minutes had passed before defender Shane Duffy made it 2-0 with a low drive.
United managed to halve the deficit thanks to Romelu Lukaku's glancing header after Luke Shaw's volley was blocked and looped back into the Brighton penalty area.
There was still time for the hosts to grab a third before the interval as Pascal Gross won a penalty - which he converted - after he was fouled by Eric Bailly. According to Opta, this was only the 10th time in Premier League history that United have conceded three first-half goals.
Paul Pogba scored a penalty in second-half stoppage time for a late consolation but could not prevent Brighton enjoying their second straight win over United.
They won 1-0 last season at The Amex Stadium in May.
In yesterday's early kick-off, Burnley slumped to a 3-1 home defeat by Watford at Turf Moor.