Bayern will host the first leg at the Allianz Arena in Munich on Tuesday and will travel to Camp Nou next week for the return leg.
"Barcelona have shown this season how vulnerable they can be at the back," UEFA.com quotes him as saying.
"However, in the crucial moments, [Lionel] Messi, [Andres] Iniesta and Xavi have always pulled out world-class performances. In my opinion, the tie will be a very open one.
"Exiting the competition would be a huge disappointment to the fans and the players after such a dominant season. It's the moment of truth for Bayern."
Kahn won the Champions League with Bayern in 2001.