Chelsea captain John Terry was taken off at half-time
The Blues boss replaced his captain with 20-year-old Kurt Zouma, with Chelsea 1-0 down at the time after Sergio Aguero's opener.
Mourinho wanted to add pace to his back-line and believes his decision was justified, despite going on to concede another two goals at the Etihad.
"It was a decision. Zouma is the fastest defender we have in the squad," Mourinho said.
"I know Man City are going to play counter attack and Zouma is a different player to John and I was proven right because Zouma was in complete control.
"John is a player I have total trust for, with me he was never substituted, with other managers he was not selected and people thought his Chelsea career was over.
"If a manager has the right, I do, because I gave the chance to make him what he is, a fantastic player and a fantastic captain.
"One point from six is better than zero and worse than two.
"I am okay in mathematics, multiply 36 by three and I know how much it gives, I know the championship is more than mathematically open."
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