Christian Eriksen fired Spurs into the lead after just 11 seconds before an own-goal from United defender Phil Jones secured victory for the home side before the break.
Defeat left United second in the table but a huge 15 points behind runaway leaders Manchester City.
In the Premier League era, Eriksen's express finish is tied for second-quickest with a Mark Viduka effort for Leeds against Charlton in 2001.
"It's a performance that starts with a ridiculous goal. That goal after a few seconds has a big influence in the game because Tottenham can play the way they like to play," United manager Mourinho told the BBC.
"In the end we played against a very good team. Both goals were ridiculous goals.
"The first goal is an accumulation of mistakes in spite of the fact that everything happened so fast. My players have watched the Tottenham kick-off many, many times," the Portuguese boss added.
- 'Kills the team' -
====================
But Mourinho said conceding an own goal had done almost as much damage to the visitors' morale as Eriksen's extraordinary opener.
"The second goal mentally kills the team. A team to win matches needs to score goals and also defend well -- on this occasion we were very bad."
In the second half, Marouane Fellaini came on as a United substitute only to be replaced just a few minutes later but Mourinho said there was a simple explanation.
"He came off because he had something in his knee," the Portuguese boss said. "Unlucky for him and unlucky for me. He came on with enthusiasm, but he has an injury in his ligament."
Spurs manager Mauricio Pochettino added: "We always try to be aggressive from the beginning.
"Our aim is to be focused from the beginning. You can win a game if you're ready to fight. It was massive to be up early in the game.
Disclaimer: No Business Standard Journalist was involved in creation of this content
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