Manchester United's players were booed off at half-time by their own fans after Erik ten Hag's tenure endured a disastrous opening 45 minutes at Old Trafford on Sunday.
The match was supposed to mark the start of an exciting new era for United under the former Ajax coach, who was appointed in April to take over from interim boss Ralf Rangnick.
But United, with Cristiano Ronaldo left out of the XI amid speculation he will leave the club this summer, produced a slipshod performance in the first half and went in two goals down to Brighton.
Graham Potter's side had lost two key players this summer in Yves Bissouma and Marc Cucurella, but Pascal Gross struck twice - after half an hour and then on 39 minutes - to give the Seagulls a healthy lead at the interval.
United's fans made their frustrations clear at the whistle as the home side left the pitch to a chorus of boos and Ten Hag will now be well aware - if he was not already - at the size of the job ahead of him at Old Trafford.
Ten Hag, having left Ronaldo on the bench initially, sent on the Portuguese for Fred after 53 minutes in attempt to find a way back into the game.
United pulled one back courtesy of an Alexis MacAllister own goal with 22 minutes remaining, but could not find an equaliser in an ultimately disappointing first home game under Ten Hag.
