The runaway Primera Liga leaders hold a 3-1 advantage from the first leg at the Calderon and are favourites to complete the job in the Nou Camp on Wednesday.
But Sunday's 3-2 comeback win at bottom club Osasuna, when they needed to score twice in the last 10 minutes to win a game they had led 1-0 at half time, was still worrying Guardiola.
"In general we played well on Sunday but we started the second half badly and this is when a team can score goals. Like Osasuna, a team can score twice in ten minutes," he told a news conference.
"It is the first time this has happened to us. As coach, it is up to me to see this and warn the team.
"If there is a dangerous side to play it is Atletico, who in the past have pulled off many unpredictable results, with a lot of goals."
He went on to point out that Atletico's three consecutive defeats last week, after a run of 15 matches unbeaten in all competitions, proved his own side's run of five wins on the trot counted for nothing.
"What happened to Atletico is a good example of what could happen to us. This tie isn't over yet."
