Pep Guardiola's side were at the height of their powers going into the semi-final, but Inter famously defeated Barcelona despite conceding most of the possession to their opponents.
Inter won the first leg 3-1 at San Siro, before holding on for a 1-0 defeat at Camp Nou in the second leg to advance via a 3-2 aggregate scoreline.
Mourinho's side would go on to win the Champions League by defeating Bayern Munich 2-0 in the final in Madrid.
But Inter's bigger feat was advancing past Barcelona and Lionel Messi, who had scored four goals in Barcelona's quarter-final second leg win over Arsenal that season.
Mourinho explained that stopping Messi, who was playing on the right wing, had to be a group effort involving defensive midfielders Thiago Motta and Esteban Cambiasso, as well as left-back Javier Zanetti.
"The situation for us was very clear, which was [Messi] cannot play alone when he comes in between the lines," Mourinho explained in a video posted to The Coaches' Voice YouTube page.
"So this player here [Cambiasso] must be a player totally in control of this area, always in communication with the left-back.
"There is a moment where he becomes yours and I stay in the zone but if in a certain moment you are attracted by other positions and Messi goes in these positions in between the lines, you have to decide to go.
"But then if you decide to go, you have to defend [Dani] Alves. So there is a combination of ideas but basically everything was around not letting Messi play."
After the game, the Italian press lauded Inter's tactical set-up, saying Mourinho's side had put Messi in "jail."
"I remember after the game ... in Italy they were using the word 'gabbia', the real translation is like a jail to Messi because in the end we didn't play man-to-man but Zanetti, Motta, Cambiasso, everybody was responsible for any position that Messi could go."