I think they lack substitutes. Every time one of the magic 5 is changed the team doesnt work the same way. And trying to win 2 titles (league and champions) without substitutes even if your main team is the best in the world isnt easy at all.
0-0 with Barcelona in the Camp Nou isnt a safe result. If the match ends 1-1, 2-2 etc in England Barcelona wins. Besides, the main flaw of Barcelona is the slow pace they have to break closed defenses. But Manchester in Old Trafford plays offensive and many players are not exclusively committed to defensive positions. Ronaldo, Rooney, Tevez, Giggs, Anderson, Evra, they all love to attack. And Barcelona has quick fowards for counterattack where Manchester isnt very safe. Messi is hard to stop with closed defenses when there is almost no room to play, in open places is almost unstoppable, and you cant put 3 men over him like most teams do if you have only 5 or 4 fixed in the back. You don't want this to happen to you
http://www.youtube.com/watch?v=hokq-7olthEThat without counting you can't leave free Etoo or Henry, you need at least a man over them, even when they are far of being in their best moments.
BTW, this is a good video for the match
Cristiano Ronaldo vs Messi, the masters of free style
.. the ball is here, not here, wait here!
http://www.youtube.com/watch?v=34irJQn0eVo&feature=related