#2 Camp Nou
It is a very intimidating stadium and a fortress of sorts. Holding down Barcelona in the first leg does not mean that they will relent when it comes to punishing a team in their own backyard. PSG suffered a 6-1 mauling several seasons ago.
This is a determined and ruthless Barcelona side that can lay to waste any team regardless of their pedigree. United may even ride their luck and sneak a 1-0 or 2-0 win at home but should expect retaliation from the Spanish champions. Lyon was a victim of a thorough thrashing at the Camp Nou last time out.
#1 Lionel Messi
The constant denominator in Barcelona's recent success has been Lionel Messi. Some people have even regarded him as the greatest player of all time. Whenever his teams have been in problems, Messi has shown the ability to change games. Can United cope with Messi and his vast array of skills?
He is the captain and main man for Barcelona. There is a potential 'easier' route to the final and he will be aware of this. What game plan will United deploy to simply tame this man? Even with a game plan, no player can say with authority that they can successfully man-mark Messi for 180 minutes.