Albion almost staged an incredible comeback from three goals down but Bryan Mbeumo’s second of the night – in the sixth minute of injury-time – made sure of the victory.
The hosts were in charge for 73 minutes, with goals from Matheus Cunha, Casemiro and Mbeumo giving Ruben Amorim’s team a three-goal lead.
But Welbeck’s customary goal against his former team – a classy free-kick – gave the Seagulls hope, with 16 minutes of normal time remaining.
With seven minutes added on, you knew there was time for two more goals. Albion got one of those – with substitute Charalampos Kostoulas scoring his first in Brighton colours.
The Greek striker nodded in at the near post from James Milner’s corner – setting up a grandstand finish.
But Mbeumo extinguished any hopes of a sensational comeback with a powerful strike into the roof of the net. It was virtually the last kick of the game.
The match was not without its controversies. There was a big first-half penalty appeal, before the opening goal, when Amad went down under a challenge from Maxim De Cuyper. Play was allowed to continue and it did not impact the hosts’ momentum.
Fabian Hurzeler will no doubt be furious with two controversial decisions against his team, though. Rutter was adamant he was fouled by Luke Shaw in the build-up the third United goal.
And then, referee Anthony Taylor decided not to send Patrick Dorgu off, after he brought down Minteh on the edge of the box – as the Brighton man looked to be through on goal. Albion scored free-kick, anyway, but Hurzeler will argue they should have been up against ten men for the remaining 23 minutes.
The result leaves United fourth in the table, whilst Albion slip to 12th.
9. Ferdi Kadıoğlu - 7
The versatile Turkish international was deployed in attacking role, with a lack of natural wingers available. Played on both flanks and made a good account of himself. Forced Lammens into a great save at full stretch at 3-1. Booked for foul on Mbeumo at 3-2. Photo: Zohaib Alam - MUFC/Manchester United via Getty Images
10. Georginio Rutter - 5
Squandered huge chance on stroke of half-time. He fluffed his lines from point blank range. Pleaded in vain for a foul in build-up to Mbeumo's first goal. Not his best performance but you can't fault his efforts. Photo: Shaun Botterill/Getty Images
11. Yankuba Minteh - 6
Brighton's biggest threat but very wasteful. Nearly scored after Messi-like run in the first-half. To his credit, he kept getting in the right positions. Looked to be through on goal when he was brought down by Dorgu. Welbeck scored the free-kick. Photo: Carl Recine/Getty Images
12. Danny Welbeck - 8
Almost guaranteed to score against Man United. Made it five goals for the season with a fantastic free-kick. Nearly sparked a crazy comeback. He was actually playing well even when Brighton were three goals down, with some lovely touches. What a player and what form he's in right now. Are you watching Thomas Tuchel? Photo: Carl Recine/Getty Images