The defender gifted Chelsea an early lead as his soft defensive head led to Noni Madueke's opener. But City eventually took control to win 3-1 on Saturday night.
Guardiola said afterwards: “What I thought was it is not the best start, right? He will take the lessons; he is young and has come for many years.
“It is not easy. I love the reaction of his mates. He made one training session. He will learn. He is young and he will take a lesson from that.
"He doesn’t speak English. In that situation there isn’t much to say, he knows he made a mistake. What can I say - don’t make a mistake! He knows it!,” the boss added.
“Marmoush, Vitor (Reis) and Khusanov cannot play on Wednesday and John needs the rhythm.
“We have a final on Wednesday with our people and John is important (which is why I made the change.)”
On the match, Guardiola also stated: “We weren’t exceptional, but we take the ball a lot and made a lot of passes.
“They don’t know the movement of Marmoush yet but in the situation, we are in a 3-1 win over Chelsea, and it is really important for us ahead of our final on Wednesday and the rest of the Premier League.”