Bayern go into Wednesday's game without a win in three matches, having also lost their first league game of the season at the weekend.
And whilst defending champions Bayern are firm favourites to progress, Guardiola is clearly taking United very seriously.
"It's a final, a life or death match," Guardiola said.
"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."
Jerome Boateng agreed with his boss, in that the United game does feel like a final.
"I've got a few butterflies in my stomach," Boateng said.
"It's like a final and we've got to play to win because we will end up losing if we play for a goalless draw.
"We've got be aggressive going forward from the start and try to score. United will sit back again, but they'll take more risks the longer the game goes on. We know what we're capable of, and we also know we'll need a very good performance if we're to go through.
"We need just a little more pace, we need to get our crosses in a little quicker, and we should sometimes try our luck with shots from long range."