Barcelona are the best team in the world, though Manchester United are fresher and have the experience you need to stop them.
Compare it to Spain in the World Cup. They need the ball, to keep possession and suck you in. There were two styles against them, the South American style with high pressure and one-v-one everywhere, and then the European, which is to drop, defend really deep and don't give them space in front of the 18-yard box.
Switzerland did that and beat them, though nine out of 10 times they would lose. Paraguay and Chile were very unlucky against them.
If you give them the ball it's tough because they've got outstanding players in one-v-one situations and then they can hurt you. Hernandez, Rooney and Valencia are the key players. Chicharito stretches the team and if anything the two centre-halves of Barcelona can be exposed. It will be a fascinating game.
Barcelona have been playing that style for 20 years because they are brought up that way. With Messi you need to double up, use two players all the time. He will play in front of Vidic and Ferdinand, then just drop to attract one of the two of them forward, so the wide players Pedro and Villa can make that diagonal run behind all the time.
So you need to defend with three and man-mark Messi. He is the only one in the world who can get the ball and go past five players. Not even Ronaldo, because he is different, it's over five yards. With Messi it's over 25 yards. So you have to stop the space but more than that, stop the service to Messi.