Bayern scored a vital away goal when the sides drew 1-1 at Old Trafford in the first leg last Tuesday.
Guardiola believes that David Moyes's side will provide a threat in the return game at the Allianz Arena.
"It's a final, a life or death match," he told ESPN.
"United will be looking to hit us on the break and at set plays and we have to be aware of that. They are one of the best teams in the world, but our away goal is very important even though 1-1 can be very dangerous."
Bayern were beaten for the first time in 53 matches by Augsburg on Saturday.