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."
Atletico coach Javier Aguirre has travelled without injured captain Maxi Rodriguez, suspended John Heitinga and Maniche, and has rested striker Diego Forlan. He said Sergio Aguero will start the match.
