I dont think Barça have the best defense either, Daniel Alves is a very attacking minded player, Puyol is overrated, Piquet is okay but nothing great, and there it goes...
And remember they played the UCL final with Sylvinho on the left, who is 35 and didnt play regularly for a long time, and Toure, a defensive midfielder, as a center back.
If Real Madrid can get their attacking players to play well, keep possession and control the game, they can do wonders even without the best defence.
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