Pascal Gross' first-half strikes were enough to keep the Red Devils out despite an own goal to half the deficit in the second half.
Bruno Fernandes failed to hit the target when he was presented with a chance early on in the game. Brighton slowly stepped on the accelerator and scored two goals in 9 minutes.
Leandro Trossard caught Harry Maguire napping and picked out Danny Welbeck, who squared the ball to find Pascal Gross at the far post. The German international calmly slotted the ball inside the back of the net to drive Brighton into the lead.
The midfielder gave Erik ten Hag further headaches by doubling the lead for Graham Potter's men in the 40th minute. Solly March shot with his left foot, attempting to find the far post corner. His shot was parried away by David De Gea into the path of Pascal Gross, who lashed on to the rebound to put the visitors in the driving seat.
Marcus Rashford had two delightful opportunities, including a chance in the second half when he failed to divert the ball into the back of the net from a long ball played by Diogo Dalot.
The hosts scored with 21 minutes left on the clock. Robert Sanchez jumped to save Diogo Dalot's initial attempt from a corner, but the ball ricocheted off MacAllister to pull one back. Old Trafford were prepared to witness a comeback under their new manager, but it wasn't meant to be.
We look at five talking points after the Seagulls created history in Manchester.
#5 Red Devils lost the midfield battle by a huge distance
Manchester United's midfield was torn apart by the visiters' midfield, comprising of Adam Lallana, Moises Caicedo, Alexis Mac Allister, Leandro Trossard, and Pascal Gross. Often, the Seagulls' midfield bypassed the Manchester United midfield and created numerous chances in the first half.
Part of why McTominay and Fred struggled to cope with the intensity of the game was the failure to track back from Christian Eriksen and Bruno Fernandes.
The creative duo rarely supported their teammates and left the backline exposed. Erik ten Hag will be hoping for a midfield reinforcement before the transfer window closes.
#4 Lack of imagination in the final third throughout the game
Manchester United struggled to play a string of passes and open up their opponents' backline, unlike Brighton. Despite consisting of creative outlets such as Bruno Fernandes, Christian Eriksen, and Jadon Sancho in the starting lineup, the Red Devils failed to show enough potency and create chances to disrupt the visitors.
On the other hand, Adam Lallana and Moises Caicedo completed 3 key passes each, while Leandro Trossard completed 2 key passes throughout the game. Besides, Brighton were mobile and gave their opponents a really tough time.
#3 Forward line of Brighton was terrifying
Danny Welbeck, upon his return to Old Trafford, was constantly tormenting Harry Maguire and Lisandro Martinez. The former Manchester United forward played as a lone striker for the Seagulls, but received an ample amount of service from the midfielders, who were on top of their game. He was also in the thick of things in the forward line and assisted Gross for the opening goal of the game.
Graham Potter's side took 15 shots and placed 4 on target throughout the game. They struggled in the second half, but the collateral damage in the first half helped them see out the game when the home team was pressing for the equalizer.
#2 Manchester United left it too late
Manchester United failed to show enough urgency and were passive throughout the first half. The Red Devils created one half chance in the first 45 minutes, which Bruno Fernandes failed to convert in the opening moments of the game.
Ten Hag's half-time talk must have invigorated Manchester United to go toe-to-toe with their opponents in the second half. The Red Devils showed intention and created a few chances during the opening stages of the second half.
Manchester United finally scored their first goal of the 2022/23 campaign through an own goal by Brighton midfielder Alexis Mac Allister. Graham Potter's substitutions helped the Seagulls control the last minutes of the game.
#1 Brighton carry on from where they left off last season
The Seagulls won their last 4 games at the business end of the previous season and finished the campaign on a high. They carried on from where they left off by producing a phenomenal display at Old Trafford to bag a thoroughly deserved 2-1 victory over Erik ten Hag's Manchester United.
Graham Potter was forced to sell Marc Cucurella and kept the remaining core team this season. Exciting times ahead for the Seagulls faithful with the additions of Simon Adingra, Julio Enciso, and Levi Colwill in the transfer window.
Paul Merson has predicted all the GW 1 fixtures of the Premier League. More details right here