Two years ago in Rome they triumphed 2-0 when, following a frantic United start, Samuel Eto'o scored after 10 minutes and Lionel Messi wrapped up the victory 20 minutes from the end.
In truth, it was a disappointing match where neither side were at their best and expectations are high for an improvement all round when they meet again at Wembley on Saturday.
"We need to play much better than we did in 2009 and this is one of the things I've told my players repeatedly in the last few days," 40-year-old former Barca player Guardiola told a news conference before his squad got a taste of the Wembley surface.
"United were better in defence and attack and we need to be much quicker. Everything was new to us really and there was a lot of chaos and confusion. We know each other a lot better now and if we play like that again we won't win."
"Both teams have had an extraordinary decade, it is very difficult to repeat the sort of success both have had, to keep making finals and winning trophies," Guardiola said.
"Whoever loses tomorrow you can't take away the achievements of the last few years."
"It's always difficult the first time round, you have to break that psychological barrier," he said. "I don't think you really enjoy finals, you have to suffer, but that was important for the club."
"There have been many great teams through history, I haven't seen them all and it's impossible to compare," he said.
"If in the next five or 10 years someone remembers the team we have now that would be good but to say we are the best team ever is impossible, it's just not true."
He took a similar line when pressed on Saturday's clash being built up as one of the best finals for years.
"Tomorrow we have to show if we deserve all this talk of it being the best final, maybe when both teams want to play and both respect the ball and want to attack it should be a good final," he said.
"We both have our strengths and we have to see who can
