The defending Premier League champions were frustrated and contained for 45 minutes at the Etihad Stadium against Dean Smith’s side.
Whatever Guardiola said at the interval, it made an immediate impact as Raheem Sterling broke the deadlock inside seconds of the restart.
Guardiola believes his side are more than capable of sustaining a challenge for a third successive title, but has also warned that they will head in the opposite direction if standards are not maintained.
He told reporters: “Six days ago we were eight points, now we are three.
“I am not concerned about the table. I am concerned about the way we played.
“If we play the level we played in the second half we can be there at the end of the season I am sure of that.
“If we play the way we did in the first half, we will be in the relegation positions far away from the leaders of the table. That is my view. Not just this season but all the time.
“The first half we play football but not in the way we should play.
“The second half we were better, an early goal in the second half helped us and we played in the way we normally are but we have to try to play 90 minutes and not just 45.”
Pressed further on the opening 45 minutes and why his side were so flat, Guardiola said: “Normally we use a word aggressive. I like to use this word aggressive when you have the ball.
“Raheem [Sterling] was the only player who was aggressive with the ball in the first half.
“In the second half we imitated him. He showed us the way we have to play and it was much better.
“The aggression is how you make movements, making the build-up quicker, how you give the pass, how you attack, how you arrive with the ball and be aggressive with the ball. We were not aggressive in the first half and in the second half it happened.”
Guardiola is delighted to see the England international ignoring the hype which continues to build around him, with the 24-year-old forward proving to be a model of consistency and professionalism.
He said: “What I like the most about Raheem was one thing: after scoring three goals against Atalanta, making an assist and winning a penalty, and being involved in the final third, everybody in the media says how good you are, normally the next game you play a little like: 'Look how good I am'. And it was completely the opposite.
“Look at Phil Foden, he played 25 minutes like it was the last 15 minutes of his life. That is our strength as a team in the last years together. Every single game, every three days.
“Today Raheem has played all the minutes, I am so proud of him.”