Erik ten Hag's first ever Premier League game as Manchester United manager ended in a 2-1 defeat at home to Brighton & Hove Albion.
Pascal Gross scored twice in the first half before an Alexis Mac Allister own goal gave United hope.
But Brighton held on to record their first ever EPL victory in Manchester.
United were booed off at half-time after being outplayed by Brighton.
Ten Hag's side had 61% of possession in the opening 45 minutes but Brighton were the much more dangerous side, registering 12 shots to United's five.
Center-back Lisandro Martinez, nicknamed "The Butcher", was making his debut for United, but he was part of a defense that was itself chopped up by some incisive Brighton passing.
With right-back Diogo Dalot caught out of position, Martinez charged across to try to stop Danny Welback's cross in the build-up to Brighton's opener.
But Welbeck still managed to deliver into the zone that Martinez had vacated, leaving Gross in space to fire into the net.
Brighton's second goal was even better. The move started near their own corner flag where United were applying a two-man press, but a confident back-heel sparked a counter-attack.
Moises Caicedo then ran past Christian Eriksen before feeding Gross, who two passes later found himself in the perfect position to slot home a rebound after David de Gea had spilled Solly March's shot.
The mood at Old Trafford lifted in the 52nd minute when Cristiano Ronaldo entered the field as a replacement for Fred.
But moments later things could and perhaps should have got a lot worse for United. Martinez, who had already been shown a yellow card, barged Welbeck to the ground inside the penalty area.
Paul Tierney said no penalty and his VAR showed solidarity by not recommending a pitch-side review. United got lucky.
It was almost 2-1 on the hour-mark after Ronaldo raced away down the right and crossed for Marcus Rashford, but Robert Sanchez made his first major save of the game. The flag was then raised for an earlier offside against Ronaldo, but replays showed he was onside so the goal would have stood had Rashford found the net.
Rashford missed another chance five minutes later when he skied high and wide following a wicked Bruno Fernandes delivery.
United did score their first goal of the season on 68 minutes and what a mess it was from Brighton's point of view.
Sanchez failed to clear from a Fernandes corner and the ball bounced back towards his goal via Dalot's arm, which had been tight to his body. Had the ball gone straight into the net off Dalot then the goal would likely have been disallowed for handball, but it did not.
In a desperate attempt to make up for his error, Sanchez slapped at the ball, only to see it hit Mac Allister and rebound backwards for an own goal.
Old Trafford was rocking for the first time all day, but it did not last.
Brighton kept cool and frustrated United for the final 25 minutes, sparking more boos at the final whistle.