Goals from Juan Mata and Marouane Fellaini saw off a patchwork Hull side on Tuesday as United put one foot in another Wembley final.
A ninth win in succession was met with a lukewarm response through much of the contest from the home fans, despite Mourinho's gesturing for the volume to be raised at half-time.
And with Liverpool arriving at Old Trafford on Sunday in a fixture between sides separated by five points in the Premier League table, Mourinho wants the crowd to fire up.
"I was disappointed yes, [Hull] were very well organised defensively as I was expecting and it was not easy for us," Mourinho told Sky Sports.
"But I think we were a bit sloppy, complicating things, always one more touch, delaying the decision, giving them time to regroup and it was not our best first half.
"I think in the first half the players have to do better, I have to do better, the fans can also do better.
"Second half we all improve just a little bit. The game is over, now I think about Sunday and on Sunday I have to do better, the players need to do better and the stadium has to do better."
Hull played with midfield pair David Meyler and Tom Huddlestone in defence as injuries bite hard at the start of Marco Silva's reign, but Mourinho did not anticipate his compatriot rolling over.
"I was expecting a difficult match," he said.
"I wasn't expecting four or five, 1-0 for me would always be a good result but 2-0 is obviously better.
"We are not at Wembley but the second goal is maybe an important goal."