Seagulls midfielder Pascal Gross netted twice in the space of 10 minutes during the first half, before Alexis Mac Allister put through his own net to halve the deficit during the second period.
Bruno Fernandes, plus debutant pairing Christian Eriksen and Lisandro Martinez, all had chances for Ten Hag's side, with the focus now quickly switching to United's next clash with Brentford next weekend.
Scroll down to see what he had to say to MUTV, plus other media outlets, following today's defeat...
AN UNNECESSARY RESULT
''Disappointed, it is clear. We of course had other expectations but that happens and this is also a process, it takes time and we have to work really hard.