Pascal Gross restored Brighton's two-goal lead from the penalty spot after Romelu Lukaku had pulled one back, and United could muster just one more goal - a last minute Paul Pogba penalty - to fall to a shock defeat.
Relive how the 90 minutes of action unfolded with Sports Mole's live text coverage below.
STARTING XI: Ryan; Montoya, Dunk, Duffy, Bong; March, Propper, Stephens, Knockaert; Gross; Murray
SUBS: Button, Kayal, Bissouma, Locadia, Balogun, Jahanbakhsh, Bernardo
STARTING XI: De Gea; Young, Lindelof, Bailly, Shaw; Pereira, Fred, Pogba; Martial, Lukaku, Mata
SUBS: Grant, Rashford, Smalling, Lingard, Herrera, Fellaini, McTominay
Chris Hughton: "I think that's what happens when you are a top, top manager. Jose is one of our best managers, if you look at the record he has of winning things, and I think when you are that type of manager, you are going to be in the spotlight far more than others and everything is going to be picked up."
Jose Mourinho: "The truth is [me and Paul Pogba] have been together for two years and a couple of weeks, and I've never been so happy with him as I am now. That's the truth. I cannot demand more from him, I cannot ask more from him."
BRIGHTON & HOVE ALBION SUBS: Button, Kayal, Bissouma, Locadia, Jahanbakhsh, Bernardo
MANCHESTER UNITED SUBS: Grant, Rashford, Smalling, Lingard, Herrera, Fellaini, McTominay