United thought they had won the game themselves when Alejandro Garnacho turned home Bruno Fernandes’ low cross 18 minutes from time.
However, that effort was ruled out for offside following a VAR check as the ball struck fellow substitute Joshua Zirkzee just before it crossed the line.
So Pedro was able to win it and give Fabian Hurzeler a dream start as Brighton manager after being left on his own to convert Simon Adingra’s far-post cross.
Former United striker Danny Welbeck had given the hosts a first-half lead with his fifth Premier League goal against his old club, then hit the crossbar with a second half header as Brighton scented blood.
United briefly took control after Amad Diallo had levelled on the hour but once Garnacho’s effort had been ruled out, the nearest they came was long-distance efforts from Scott McTominay and Casemiro, neither of which found their target.
The winner created a party atmosphere at the final whistle, with Hurzeler, about whom there has been so much doubt following his appointment at the age of just 31, clearly elated and Billy Gilmour, who has been linked with Napoli, named man of the match, to more cheers from the home fans, who want the Scot to stay.
Welbeck shines against his old club
Every time Brighton change managers, it seems some feel it will be the trigger for a fall from grace.
The critics were wrong when Graham Potter replaced Chris Hughton, incorrect when Roberto de Zerbi took over after Potter left for Chelsea and they were out again when De Zerbi quit and Hurzeler came in.
Each time, the Brighton model proves durable enough to cope.
Clearly, it is far too early to make a judgement on Hurzeler. But after hammering Everton on the opening day, the German could not have had a better start.
There was an element of luck involved in this success given Zirkzee only got in the way of Garnacho’s effort because he was unable to halt his slide towards goal after he had missed Fernandes’ cross.
But there was plenty to like about Brighton and with Rutter and Denmark midfielder Matt O’Reilly, who should complete his £25m move from Celtic this weekend, to fully integrate, there is plenty of improvement expected.
Welbeck remains a key element though. The 33-year-old signed a new contract in May that will keep him at Brighton until 2026.
He remains a potent threat and still has the speed and physicality to cause most defences problems.
Welbeck clearly likes playing against his boyhood club judging by the number of times he has scored against them.
His latest effort owed much to Harry Maguire opting against taking the risk of turning a low cross into his own goal, which allowed Kaoru Mitoma to collect a loose ball at the far post and deliver a return pass between a gap in defenders to give Welbeck a tap-in.
Hit and miss for Amad on bad day for United
In January, Amad Diallo will have been a United player for four seasons. The time is surely coming when a decision will have to be made about whether the 22-year-old is good enough for the level United aspire to. This was another of those days when there was evidence for and against.
A £19m signing from Serie A side Atalanta, Amad’s stand-out moment as a United player to date was his last-minute extra-time winner against Liverpool in the FA Cup quarter-final last season. In Los Angeles this summer, he spoke of his desire to remain at United for life.
With Alejandro Garnacho returning to club duty late after Argentina’s triumphant Copa America campaign, Amad has been given the opportunity to make an impact.
In a first half they had the better of but still ended up behind, the visitors failed to capitalise on their best two moments and Amad was the culprit.
The Ivorian had space to finish off an admittedly difficult chance when Diogo Dalot swung over a cross from the left but he sent his effort harmlessly wide.
Then, as United streaked forward at a counter, what should have been a straightforward pass to send Marcus Rashford racing through on the Brighton goal was delivered behind the England man, which neutralised the opportunity.
If that was the negative, his equaliser was the positive.
Set free by Noussair Mazraoui, who is excelling at right-back, Amad drove at the Brighton defence, cut inside the box and let fly with a low shot that looked to be heading for the bottom far corner before Jan Paul van Hecke slid across and turned the ball into the net in his effort to clear.
That moment alone probably explains why manager Erik ten Hag is prepared to let Jadon Sancho leave for Juventus, if the Serie A giants can come up with a suitable deal.
Yet it doesn’t really fully answer the question about Amad and nor does the mature manner in which he set Bruno Fernandes away to create what United thought was the winner.
And the outcome does nothing to convince observers that Ten Hag is making the right decisions.
