It was a dismal display and the visitors took full advantage as Pascal Gross scored twice in the first half to confirm the Seagulls’ first-ever win over United.
Ronaldo, who started on the bench, was introduced before the hour but failed to inspire the comeback.
There were more loud and angry protests against United’s owners, the American Glazer family, before kick-off, and it was also a familiar scene on the pitch.
Erik ten Hag and United take on Brentford and then Liverpool.
/ REUTERSBrighton took the lead on the half hour mark with Leandro Trossard finding former United forward Danny Welbeck sending the ball to the back post for Gross to go home.
Nine minutes later, Gross doubled the lead, with his sixth goal against United, after David De Gea badly pushed a Solly March drive into the path of the German who again made no mistake.
Pascal Gross scored twice in the first half.
/ APTen Hag introduced Ronaldo in the 53rd minute and the Portuguese forward, who is reportedly looking to leave the club, provided the spark for United’s attempt to come back.
The home side gained a foothold in the 68th minute when a goalscorer ended with Alexis Mac Allister turning the ball into his own net, but Brighton held on to a second straight win against United, following their 4-0 win in May.