The Spanish manager has likened his Champions League final opponents to the Catalan club's biggest rivals, believing they are "extraordinarily strong" and will be tough to beat
The match is a repeat of the 2009 showpiece in Rome, when an outstanding Barcelona display culminated in a 2-0 victory thanks to goals from Samuel Eto'o and Lionel Messi.
While the Catalans won on that occasion with seeming comfort and ease, Guardiola is expecting a much more difficult match against Sir Alex Ferguson's side this time.
"Manchester United are extraordinarily strong, very similar to Real Madrid," Guardiola, who has won the league title in each of his three seasons in charge at Camp Nou, told reporters.
"They are a hard-working side and it's very hard to hurt them."
The Barcelona coach also admitted he is concerned by United's counter-attacking ability, with the pace of Antonio Valencia, Nani and Javier Hernandez being key weapons for Ferguson's side this season.
"We have to be very careful because they can score against you on the break," he said.
"We have to know how to interpret the final, depending on how they play, and play the game on our terms."