The Portuguese coach was sacked by the Red Devils last December after a poor start to the new Premier League season left Manchester United off the pace in the race for the top four.
Manchester United brought in Ole Gunnar Solskjaer as the club’s new manager on a temporary basis and then handed the Norwegian the job permanently earlier this year.
Mourinho is now back in management after having taken over at Tottenham Hotspur last week following the sacking of Mauricio Pochettino.
The Portuguese coach was quizzed about the end of his time at Manchester United following his appointment at Spurs, and the former Real Madrid boss gave an honest answer about how things panned out.
Speaking in an interview with Des Kelly for BT Sport, Mourinho said: “I have to admit yes (I did lose passion at Manchester United).
“When you are not happy, people feel it. That happiness that you can influence to be better and more enthusiastic yes, I have to admit it, yes.”
Mourinho won the Community Shield, League Cup and Europa League during his spell in charge at Manchester United.
Mourinho led his Tottenham side to a 3-2 win over West Ham United in the Premier League on Saturday afternoon in his first game back in charge in the English top flight.
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