Barca, like United, have already wrapped up their domestic title and can now focus on the London showpiece on May 28.
The match is a repeat of the 2009 final, which Barca won 2-0 after a dominant display which left United manager Sir Alex Ferguson scratching his head in Rome. But Guardiola expects a more difficult game this time around.
United front: Guardiola has been impressed with the English champions' resilience
'Manchester United are extraordinarily strong, very similar to Real Madrid,' he said. 'They are a hard-working side and it's very hard to hurt them."
The Barca coach also admits he is concerned by United's counter-attacking ability.
'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.'
Guardiola picked an experimental side in Sunday night's Primera Division match at home to Deportivo La Coruna and watched his young charges draw 0-0 with the relegation-threatened Galician outfit.
'The young players did well,' he said. 'I am happy with them and I chose to rest a lot of the others because the most important thing now is to prepare for the final.'
Of the regular first-team starters, only keeper Victor Valdes, midfielder Javier Mascherano and full-back Eric Abidal were included from the outset, while Dani Alves appeared as a second-half substitute.
Wembley way: The Barca boss is wary of United's counter-attacking threat
Lionel Messi and Xavi were unused substitutes, while Andres Iniesta, Carles Puyol, Gerard Pique and Sergio Busquets all watched on from the stands.
Busquets received good news today when he was cleared to play in the Champions League final after UEFA's Control and Discipline Committee the decided against charging the midfielder for an alleged racist remark made to Madrid's Marcelo in the sides' ill-tempered semi-final first leg at the Bernabeu.
Meanwhile, Barca legend Johan Cruyff, who led the Catalan club to their first European Cup at Wembley in 1992, believes their presence in a second final in three years and a semi-final appearance last season confirm their dominance on the continent - win or lose in London.
'Whether you win or lose at Wembley, your presence in two Champions League finals, as well as a semi-final (last season), corroborates the football hegemony in the league and abroad,' the Dutchman wrote in his weekly column in Catalan newspaper El Periodico.
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Explore more:People: Alex Ferguson, Andres Iniesta, Johan Cruyff, Gerard Pique, Victor Valdes, Xavi, Javier Mascherano, Pep Guardiola, Lionel Messi Places: Madrid, Barcelona, Rome, London