United controlled most of the first half, but failed to score, with Romelu Lukaku guilty of missing an open net.
However, the game turned on its head in the second half, with Spurs scoring twice in as many minutes through Harry Kane and Lucas Moura.
Moura added a third with few minutes left, to kill off the tie.
“We work very very well during the week, we prepared the game very well and we played very well. At half time the result should be 2-0 or 3-1 to us and then something changed the game a little bit.
“The third goal came and killed the game. We worked all week, we didn’t lose on the tactical point of view, but all our fans don’t read papers, they don’t watch television, they’re more intelligent than that. Our fans answered in an amazing way.
“Goals are an amazing vitamin and goals conceded are an overdose of fatigue,” Mourinho told Sky Sports after the match.