Grealish opened the scoring for City after having been criticised for a lacklustre performance in the Champions League win over Borussia Dortmund, as well as a lack of goals and assists since his £100 million transfer from Aston Villa last year.
"I've always said if you are here, it's because I trust them unconditionally, all of them. I know them," Guardiola told reporters after the win.
"He scored a goal and it was a good performance. Every time he has the ball it's one-against-one against the full-back all the time.
"OK, he didn't score or assist (in previous games), but it's a question of time," he said. "Of course I am happy for him, for the victory, for his incredible personality to play."
Guardiola added that he liked what he saw of the partnership between Erling Haaland and Julian Alvarez, with the pair playing side by side towards the end of the match after Alvarez was brought on in the 71st minute.
"We have two prolific strikers, and this was the second time I've seen Erling and Julian together, in the last minutes like against Bournemouth, and I like it," Guardiola added. "They can be a little bit more aggressive, they have a sense for goal.
"It's nice with two important players leaving (Raheem Sterling and Gabriel Jesus), to add and still have the tendency to score goals and create a lot of chances.
"We have that feeling that when we change the gear, we can create a lot of chances in just a few minutes."
