City fell 2-1 at Villa Park today, with Guardiola believing things changed when Stones was forced off.
He later said: "We make a really good first half, second half we dropped. We changed our pressing for the reason John (Stones) could not continue. Congratulations Aston Villa and keep going.
"n the mid-block they are so strong. We have good moments, we had chances, more chances than in the United game, but in the second half we dropped and our pressing was not good enough. We struggled a little bit and we could not drop them well. We found a goal in the end, but too late.
"We struggle to score and we concede goals."
On City's goalscorer Phil Foden, he said: "The last one or two games, he has been better. Most of the players have had problems. Sometimes the next season is not the same, the people change and the situation changes. Today we made a really good game."
On whether he has to change his approach, he added: "Yeah. I have to do it. Back my players, that is what will be different."