Marcus Rashford's double either side of a superb Dele Alli goal meant Mourinho's Tottenham lost 2-1 in the Premier League, ending his 100 per cent record since succeeding Mauricio Pochettino.
Spurs lacked intensity but Mourinho rued a penalty needlessly given away by Moussa Sissoko that allowed Rashford to score what proved the winner early in the second half.
"I think they [United] deserved the victory by their first half, we didn't start well at all," Mourinho said. "I think for the first 30 minutes they were the best team and because of that, a strong 30 minutes, we were not there. Also, the way we conceded the second goal, I think they deserved to win.
"If I look even to the previous three matches where we won, we didn't start well. The team is not starting matches well, finding it a little difficult to find the intensity, the aggression of the game.
"I told them about Old Trafford, about some of the players like [Scott] McTominay who show aggression immediately in the game, Rashford even going to press the keeper, this type of attitude showing initially intensity, but we were not in the game in the beginning.
"After that, we had the ball, played and scored and we could create even more, then in the second half I thought really we could be in control of the game like we finished the first half.
"We conceded a goal that is impossible to concede if we are to win, we are sleeping, we have no control. Then what happened inside the box is a consequence of everything that comes from the sleeping."
Spurs will look to bounce back at home to Burnley having seen United leapfrog them in the table and move into the top six.
"I need to work because until now I almost didn't work, with two days between matches," Mourinho, sacked by United a year ago, said. "Everything is very passive, not like intense training sessions, it's about recovering and trying to give tactical principles. But we need to work, they [the players] feel exactly what I feel, they didn't start the game.
"Then we were lucky to finish the first half at 1-1 then, when we thought we had the best conditions in the second half to be in control and try to win it, you concede that goal and it is not a good one to concede."