It was a dismal display, and the visitors took full advantage as Pascal Gross twice scored in the first half to confirm the Seagulls’ first ever win over United.
Ronaldo, who started on the bench, was introduced before the hour mark but could not inspire the comeback.
There were yet more loud and angry protests against United's owners, the American Glazer family, before the kick-off and it was also a familiar scene on the field.
Brighton took the lead on the half-hour with Leandro Trossard finding former United forward Danny Welbeck who squared the ball to the back post for Gross to slot home.
Nine minutes later, Gross doubled the lead, with his sixth career goal against United, after David De Gea had poorly pushed out a Solly March drive right into the path of the German who again made no mistake.
Ten Hag introduced Ronaldo in the 53rd minute and the Portugal forward, who is reported to want to leave the club, provided the spark for United's attempted comeback.
The home side gave themselves a foothold in the 68th minute when a goalmouth scramble ended with Alexis Mac Allister turning the ball into his own net but Brighton held on for a second straight win against United, following their 4-0 victory in May.