United have been consistent rather than spectacular this season in winning the domestic league, while on the other hand Barcelona continue to receive plaudits for their passing football and the question has been posed whether this is the best team of all time.
However, 40-year-old Guardiola - who was part of the Barcelona team that won the club's first European Cup at the old Wembley in 1992 - was having none of that. "I saw United play in the second leg of the (Champions League) semi-final against Schalke and they were very strong even without several important players," said Guardiola, who in his first season in charge in 2009 guided Barcelona to the treble of league, cup and Champions League.
"They have a lot of depth and are capable of four or five different line-ups without any problem. They are always competitive and are one of the best teams in the world and have been for the last five or ten years."
The game in London is a rematch of the final in Rome two years ago when Barcelona came out on top 2-0. United had a good opening spell but then after a goal from Samuel Eto'o the Catalan side took control and dominated with Lionel Messi sealing the win in the second half but Guardiola said there was much from that match to absorb.
"We have been looking again at that game and I think that Manchester were much better than us in the first half and then we were the better side after that. I think it is now going to be another tight match," said Guardiola.
"I would say it will be a good final but you never know as sometimes players are scared with it being a final. We are up against (Wayne) Rooney, (Antonio) Valencia and Chicharito (Hernandez) who are very good players that can pressure us a lot and are very dangerous in space. On top of that though they have a strong defence.
"(Coach Sir Alex) Ferguson has been at the top for many years and is one of the great names of football. In the United team there are still a lot of top names that were in the final in 2009 although (Cristiano) Ronaldo has gone. Ronaldo is an important player but they are still a powerful team."