The 43-year-old sympathised with David Moyes for the position his team are in but believes the Red Devils are still one of the best teams in the world
The English and German champions meet at Old Trafford for the first leg of their quarter-final tie in starkly different circumstances, with United languishing in seventh place in the Premier League while Bayern have romped to the Bundesliga title in record time.
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"Manchester United are one of the world's best teams," the Spaniard told reporters in his press conference. "It doesn't matter what situation they're in right now.
"They have good players. These two games could be the best two performances of their season. They can beat us.
"They are one of the world's most important clubs. They have great players, it's an honour to be here."
The 43-year-old insists he has sympathy for Moyes for the pressure he is under in his first season in charge of United.
"I could also be in his situation one day. Football is a fast-moving business these days," he said.
Guardiola went on to stress that the Bundesliga remains the most important competition for Bayern - but admitted he would be "honoured" to beat United en route to defending the Champions League crown.
"The Champions League is not the most important title in the season," he replied. "The local title is the most important. The Champions League is the most prestigious. We will try but we know how difficult it is.
"At a coach, it's important to maintain the success. We won it last season and we look to progress. To do it against Manchester United is a real honour."