The Catalan giants head into the match 1-0behind on aggregate following Didier Drogba's first-leg strike.
However, Guardiola is confident that his team is capable of responding to the loss by raising their game at the Nou Camp tomorrow night.
"I don't know if this is the most important moment of my career as a coach, but I have always faced these challenges with a lot of optimism as I believe in my players and team," he said.
"Whenever you have a defeat, the day after the match you feel a little low but afterwards you get faith in the facts and I am optimistic because I know them well and that we will work hard."
Barca were handed a boost earlier with the news that Lionel Messi is back in training following a gastric issue.