Chelsea took the lead against 10-man City - who had full-back Pablo Zabaleta sent off in the 66th minute - as Mourinho's decision to bring on Andre Schurrle for Ramires paid dividends.
Germany forward Schurrle netted just five minutes after Zabaleta's dismissal to nudge the visitors ahead, and their advantage came agonisingly close to being extended as Diego Costa hit the post.
Chelsea's profligacy was then punished by their own all-time top scorer, as Frank Lampard - who left Stamford Bridge after 13 years in the close-season - scored five minutes from time to earn a point for City.
And Mourinho felt that Chelsea's inability to build on their lead was pivotal as his side were denied a Premier League win for the first time this season.
"Because we hit the post," Mourinho replied when asked why Chelsea failed to maintain their 100 per cent record.
"We had a 1-0 lead and when you don't kill matches, you are in danger - especially when you play against very good teams."
The Portuguese was also complimentary of City's spirit and conceded that the hosts deserved a share of the spoils.
"We had a tactical reaction to try to win the game, they had an emotional to try not to lose the game," Mourinho added.
"We were successful in our tactical change to try and win the game because we scored and had more chances to score. And they were successful in their emotional reaction and I think in the end it is a point that they deserve."
