United were under the guidance of Sir Alex Ferguson at the time and Mourinho believes he found the perfect approach to deal with the Red Devils.
Having arrived in English football during the summer of 2004, a daunting opening-day test was handed to Chelsea’s new boss in his first competitive outing.
The Blues were to edge United 1-0 at Stamford Bridge and went on to break their title duck by claiming the crown with a 12-point advantage over the chasing pack.
Mourinho believes he put down a marker in that first game against United, telling The Coaches’ Voice as two old adversaries prepare for another opening weekend clash in 2019-20: “It was important to win against a direct rival.
“In the context of 38 matches you cannot lose many points, especially at home against direct rivals if you want to win the title.
“I thought it was very important because there was not a contradiction between our words to the players, our ambitions, and then the result of the match.
“If you say ‘this season is to be champions’ and then the first match you lose 3-0 at home to a direct rival, you give one step back.
“So it was one step forward, there was a big relation between that match and what we tried to give to the team in pre-season which was mental strength, being compact, tactical discipline and then be really ambitious on that approach that sometimes I call the ‘all in approach’.
“You have to put everything in that game, in every minute of the game, every second of the game, it’s all in.
“It’s your qualities, your physical power, it’s your aggression, it’s your emotional control, you have to put everything in.
“That game was one of those matches where we could feel the ‘all in’.
“It’s one of those matches where I felt the game is 90 minutes, but it could be 900 minutes and they wouldn’t score. We would always win the match.”
United, who are now working under the guidance of Mourinho’s successor Ole Gunnar Solskjaer, are set to welcome Chelsea to Old Trafford on August 11.
As things stand, the Blues have no manager at the helm, with Frank Lampard being heavily linked with a return to west London following Maurizio Sarri’s departure for Juventus.