Following a hard-fought 1-1 draw in Belgium last week, United required an extra-time winner from Marcus Rashford at Old Trafford to get past Rene Weiler's Purple and Whites 3-2 on aggregate.
After the match, Mourinho spoke of the difficulty his players had in navigating the Belgians to advance to the semi-finals.
"It was a difficult game, a difficult opponent, I am tired, I imagine the players are more tired than me but we are in the draw tomorrow and we still have the dream to win the competition," an exhausted Mourinho said.
"I think we had like 17 shots and 10 on target, something like that but the game was difficult.
"When you look at these numbers, maybe somebody who didn't watch the match thinks it was total dominance but it was not like that. They started much better than us and despite our chances, they had good control of the game for about one hour. I think when we put one more player in midfield we started controlling the game and then you are always at risk because at 1-1 you are at risk.
"But from that moment we were in control and it was just a question of scoring and not conceding. It was hard though."