Manchester United Football Formation

Lineup (3-5-1-1)

The team I would play to beat Barcelona.

DEFENCE:
Press Ferdinand and Smalling right up into Xavi and Iniesta's faces (switching between man marking and zonal, should Xavi/Iniesta roam deep) and not give them time or space to pass.
Evra should man-mark Pedro.
Park should man-mark Villa.
(Evra and Park must switch sides, if Pedro and Villa switch, man-marking is strict here, unlike that of Ferdinand and Smalling).
Vidic is the sweeper and should always be aware of gaps and protect the spaces, should Smalling or Ferdinand get beaten. Vidic must fill the gaps, but must NOT get sucked onto the wings, he must remain rather central. Also, he must not proceed too far with the ball. He should try to dispatch to either wing or use Scholes as an outlet. Vidic's main marking responsibility arises if Messi reaches him (this will be explained later).
Scholes has no marking responsibilities (with an exception, mentioned later), and must drift around defensive-midfield looking to play pinpoint passes to the wings. He must utilise passing, rather than dribbling and must sit in-front on Ferdinand and Smalling, as an outlet. Here is the exception to Scholes' marking: the aim of this formation, is to isolate supply to Messi, from Xavi and Iniesta. Hence, Messi will have to drop very deep to pick up the ball. It is now Scholes' responsibility to hound Messi into making mistakes and misplacing passes. Hopefully, from this deep, Messi will have to cover too much ground to arrive at goal and will be stopped along the way. If he reaches the box (or just outside the box), Vidic must now step forward and shut him down. This must ideally be done with Scholes and Vidic doubling up on him.
Nani will be required to quell Dani Alves' bursts down the wing, and Scholes may be used to help the cause.
Abidal doesn't do much going forward, so Scholes should look to position himself towards Alves' side (and help Nani, should it be needed).
Valencia, if this happens (when Scholes positions himself towards Nani's side), MUST still remain wide right, as an outlet. If Scholes is sucked towards the left too far (helping Nani), Rooney must fill the gap and stick to Busquets. Otherwise, Rooney needn't mark Busquets.
Hernandez has no marking responsibilities, but will be required to put pressure on Puyol and Pique and force them to pass to Valdes.

OFFENCE:
Park and Evra must really press FAR up the pitch, overlapping Valencia and Nani.
Now, Smalling and Ferdinand must drop deep, forming a 3-man defensive line (Smalling, Vidic, Ferdinand). Scholes must also sit back as a defensive midfielder.
Nani must use his trickery to stretch Dani Alves' legs, because you can't beat Alves for pace, you'll need to use skill and trickery. Hopefully, by making Alves defend every now and then, he won't have the energy to work the wings for 90 minutes.
Valencia must look to power down the wing and get crosses in, and really test Abidal.
Rooney must remain central to this attack, play in-the-hole and link-up with Hernandez. Rooney must drift around the spaces and stretch the Barcelona defence out of shape, giving space for Hernandez.
Hernandez must do what he does best and sniff around every opportunity for goal. Every time Scholes or Rooney picks up the ball, Hernandez must look to run in-behing Pique and Puyol; they are quite slow.
Otherwise, if Nani or Valencia has the ball, BOTH Rooney AND Hernandez must flood the box, and wait for the cross. Scholes will sit deep, ensuring gaps don't form.
Evra and Park must be alive to the counter-attack and look to rush back to their markers (Villa and Pedro), in which case Scholes will drop deep again, and Ferdinand and Smalling will press up and resume marking Xavi and Iniesta, not giving them the space they want.
Vidic will become sweeper again.

I realise this is a defensive formation and tactics, but I think Xavi and Iniesta are key to every Barcelona attack. If they are pressed properly, Pedro and Villa can't receive the ball, because they play so wide. This leaves Messi to run back and start attacks himself, which will wear him out.