Yoane Wissa had the visitors ahead inside the first minute before City fought back to win 2-1 thanks to Erling Haaland's brace.
Guardiola later said: "I am a big fan of Thomas Frank. I don't remember in eight or nine years have a team playing the way they played. We were lucky with the deflection because they deserved 1-0.
"We struggled a lot to read when to shoot and press. We were lucky to find the goals but in the second half we were better and probably deserved to score more goals.
"We needed that to feel how difficicult everything is, how difficult the Premier League is every game.
"People say City so strong but the Premier League is so difficult. Today was the real proof.
"I am a big fan of Brentford. They play long balls but they have ability to play as well. I give a lot of credit to my players, from where we come from in the past but to still be able to suffer. I prefer to win that way than four or five nil. We need that."
On Haaland, he added: "Erling in the space is so difficult to control. It was a tough week for him but yesterday it was a bit better, we tried to help him."