Glenn Murray and Shane Duffy put the hosts two goals up, before Romelu Lukaku pulled one back for United.
However, Brighton scored again through Pascal Gross. Paul Pogba’s 90th minute penalty, was not enough as Mourinho’s men lost at The American Express Community Stadium.
“Too many mistakes and we were punished for those mistakes. I think the third goal is too hard and the team at half time tried but I think mentally the accumulation of mistakes and punishment was giving happiness to a good team like Brighton.
“When I speak about individual performances and am critical about players you don’t accept it and are critical of me so please don’t ask me to go in that direction, it’s not good got me.
“I was not expecting big mistakes because we aren’t speaking about small mistakes. We made big mistakes and were punished. When you lose your confidence there is a normal tendency to lose your direction of the game plan.
“We scored a goal and thought about going to the second half on a high but the third goal made us feel really really down. Our second goal arrived too late,” Mourinho told Sky Sports after the game.