Holders Bayern are firm favourites for the first leg of the UEFA Champions League quarter-final, despite it being played in Manchester.
The Bavarian giants have swept aside all before them this term, with the Bundesliga title already sealed in record time.
They have lost just once in 41 outings this term and face a United side who languish seventh in the Premier League and have lost seven home games in all competitions this season.
Guardiola is wary of David Moyes' side though, and says the firepower at his counterpart's disposal - such as Wayne Rooney and Juan Mata - could result in a surprise.
He said: "Manchester United are one of the world's best teams. It doesn't matter what situation they're in right now.
"Manchester United have good players. These two games could be the best two performances of their season. They can beat us."
Guardiola was also asked about the future of midfielder Toni Kroos, who has been linked with a move to United.
The 24-year-old is a regular for both club and country but has not agreed to an extension on his contract, which expires in July next year.
The Spaniard was keen to refute the talk linking Kroos to Old Trafford, though, and said, extension or no extension, that he will play his football for Bayern next season.
Bayern goalkeeper Manuel Neuer added United's poor league position does not mean they are ripe for the picking - using the example of Chelsea, who beat the Germans in the 2011-12 UEFA Champions League final despite struggling domestically.
"We know just how well Manchester United can maintain their focus," Neuer said.
"The name Manchester United deserves a lot of respect alone. They fought back well against Olympiacos (in the round of 16).
"Think of Chelsea who finished sixth and went on to win the Champions League - unfortunately."
Bastian Schweinsteiger, a key member of Bayern's midfield, shared a similar view.
"It's something very special to be able to play at Old Trafford," he said.
"Everyone knows what to expect tomorrow… it will be an incredibly difficult game."
