Chelsea boss Jose Mourinho says the need of having a fast defender on the pitch was the reason why he took John Terry off at half-time.
It was the first time the Englishman was taken off at half-time under Jose Mourinho's management in the Premier League.
The Blues went on to lose 3-0 away from home against Manchester City, one of their heaviest defeats in the domestic league in recent seasons.
"I had to decide on John or Cahill because Zouma is the fastest player we have in our defensive line," Mourinho told reporters.
"When you are going to play with a high block and you are asking your players to do that, the fastest player has to be on the pitch.
"The point was not to bring John out but to bring Zouma in.
"I wanted to have my fastest player on the pitch and not on the bench because I knew they would play counter attack and long balls into Aguero. I knew I wanted my team to be dominant and to play in the opposition half.
"I'm the one that plays John every game, made him captain, brought him back from a difficult situation with other managers.
"I'm the one who has the right to look at the game and say I want Zouma on the pitch. I was losing 1-0, in minute 35 I was trying to anticipate what I was going to say to the players, and ask the players at half-time, and immediately I had Zouma warming up because it was clear to me he had to play."