Grealish suffered a dead leg in a win over Sheffield United in August which kept him out of two games last month. Since his return, the England international has struggled for game time, starting on the bench in City's last four Premier League matches.
"I don't have one second or piece of doubt about Jack and his quality and what he has done for us since he arrived two years ago, and especially last season," Guardiola told reporters ahead of Wednesday's game in Bern.
"He needs rhythm - he's not like (Phil) Foden or Bernardo (Silva) who need two or three training sessions to be ready. He's incredibly focused to get back and we're going to use him and we believe he will help us.
"He knows and everyone knows there is competition. The group of guys we have, the relationship is beyond exceptional. But they have to convince themselves they are ready. Sooner or later the opportunity comes, and you have to be ready."
City will look to maintain their 100% winning record in Champions League Group G when they take on Swiss side Young Boys, but the European champions will have to contend with an artificial pitch at the Wankdorf Stadium.
Guardiola said City had travelled to Switzerland earlier than usual in order to get accustomed to the surface, adding: "UEFA allow (games) to be played here because it is in good condition.
"That's why we travel in the morning, so the players feel how the ball runs right, left, backwards, forwards. That's why we try it and the players will know it immediately.
"Grass is better; 99% of teams in high-level play on grass."
