Guardiola: Leverkusen will pose physical test
"They are a physically a stronger team than us and they will be stronger than us when we do not have possession," Guardiola told reporters ahead of Tuesday's first leg in Leverkusen.
"They are dangerous from set pieces and dead ball situations and they are very strong in the air."
Leverkusen are the last team to beat Barcelona in Germany back in the 2001/02 season.
Guardiola said Barca needed to control possession against a side who won all three of their home group stage games, including successes against Chelsea and Valencia.
"My biggest concern is knowing Leverkusen well," added the coach. "I know their strength and their virtues. They are a German team.
"If we do not control possession then we will have problems. "People think that because the league is difficult we will simply win the Champions League and if we do not win it then it is a failure."
"Our goal is to compete well and see how far we get. We will take stock at then end of the season."
Barcelona stumbled to a 3-2 defeat at Osasuna on Saturday that left them 10 points adrift of leaders Real Madrid in La Liga.