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The atmosphere at United seemed to improve over pre-season, but its performance on Monday (AEST) was little better than the underwhelming football regularly served up under Ralf Rangnick.
United struggled to cope with Brighton's fluidity and the visitors deservedly found themselves two up at half-time as Pascal Gross – twice the scorer of winning goals against the Red Devils – netted a brace.
Cristiano Ronaldo – benched due to a lack of fitness – made a difference in the second half and a scrappy Alexis Mac Allister own goal halved the deficit, but Brighton held on for a first ever win at Old Trafford.
There were warning signs inside 20 seconds when a Diogo Dalot error led to Leandro Trossard smashing into the side-netting.
As Brighton's confidence continued to grow, Danny Welbeck inspired the opener with half an hour played, flashing the ball across goal for Gross to tap in at the back post.
Gross was in the right place at the right time again nine minutes later, beating Fred to a rebound and converting after David de Gea parried Solly March's shot.
Ronaldo's introduction just after the break did little to turn the tide initially, with United lucky to avoid a penalty concession when debutant Lisandro Martinez barged Welbeck over.
The Portugal striker did lay a great chance, though, picking out Marcus Rashford who was denied at point-blank range by Robert Sanchez.
But Brighton's goalkeeper was at fault as United pulled one back, putting Mac Allister off as he tried to clear off the line.
United's late flurry failed to produce an equaliser, however.