Manchester clubs’ mixed fortunes in recent seasons continued as United suffered another false dawn in Ten Hag’s first competitive game in charge.
Cristiano Ronaldo started on the bench for the Red Devils as he didn’t train before the season as the Portuguese reportedly wanted to leave Old Trafford to play in the Champions League this season.
Brighton beat United 4-0 when the teams last met in May and although the Seagulls have lost Yves Bissouma and Marc Cucurella for more than £100m ($121m) they were once again too skilled for United’s lead defence.
Twice in nine minutes, Pascal Gross broke through at the far post to slip into an unguarded goal before half-time.
It even took a Brighton error for Ten Hag’s men to find the net when Robert Sanchez smashed the ball into his own net past the hapless Alexis Mac Allister.
Ronaldo received a rousing welcome from an otherwise frustrated crowd but never had a clear sight of the goal to deny Brighton their first win at Old Trafford.
“It’s the worst possible start to be down 2-0 at half-time at Old Trafford,” said United captain Harry Maguire.
“It’s not good enough. We huffed and huffed to get one back and we had chances to equalise. It’s a bad start for us.”
Brentford managed to salvage a 2-2 draw with Leicester as they trailed two goals.
The Foxes were in pre-season turmoil as longtime captain Kasper Schmeichel joined Nice and rumors swirled about the futures of James Maddison and Wesley Fofana.
Both were in the starting XI and Maddison’s corner met Timothy Castagne, who scored the opener.
Kiernan Dewsbury-Hall’s superb long-range effort doubled Leicester’s lead early in the second half.
But Brentford bounced back to take a point when Ivan Toney used his power to fire from close range before Josh Dasilva found the top corner.