Rio Ferdinand & Nemanja Vidic vs Leo Messi and negative space.
No doubt Ferdinand and Vidic are one of, if not the best central defensive partnerships in world football, and can repel aerial bombardment and fluid, incisive, attacking pressure. Leo Messi, David Villa and Pedro are something entirely different. Their combined interplay in and around the penalty box causes havoc in regards to marking. Neither man to man, nor zonal marking work against their front three in open play, as Messi, operating as a central floating, "false nine" attacker, creates space just in front of the opposition penalty box
for Pedro, Villa, Iniesta and Xavi to run into. He also drops of the defensive line to turn and run/dribble at the defenders, which catches defenders on their heels as his agility, speed and close control draws fouls with remarkable frequency.
Without the ball, they close down like a pack of hounds, and if they don't get the ball back within a few seconds, they fall back into defensive shape, without loosing easing the pressure off those on the ball.
They don't just pressurize the opposition defense when out of possession, but they pressurize in such a manner as to force defenders into giving the ball to the player least comfortable on the ball, who they then force into an error.
2 or 3 Midfielders vs Xavi, Iniesta & Busquests(Mascherano)
Carrick is most likely to start, but who he'll be partnered by is a mystery, most journalists predict that Ryan Giggs will start, as he started against Chelsea and Schalke in earlier rounds. Whether or not Sir Alex opts to play a third central midfielder, there is no doubt Park and Valencia on the wings will tuck in when they have not got possession, and that Wayne Rooney will drop back to mark Busquets and attempt to distrupt the flow of balls from defense to Xavi and Iniesta.
Wayne Rooney or/&Javier Hernandez vs Carles Puyol & Gerrard Pique
Pace, finishing accuracy and pressure. There is no doubt that United will create a few chances during the game, and these chances may come early, or may come in the final seconds of the match. Either way, Rooney, Hernandez or whoever gets the chance must take it. In 2009, if Park's shot had gone in past the protruding foot of Gerrard Pique, the nature of the game would have changed completely. United cannot afford to miss golden opportunities, whenever they come. Hernandez is a great harrier, and will apply pressure on Puyol, Pique and Valdes, forcing them into playing long balls under pressure from defense will help United, and United's defence must also be wary not to concede possession in a similar manner, something Vidic, Ferdinand and Carrick sometimes do. Shooting from distance can work against Victor Valdes, who can be caught cold from 18/20 yards with well placed shots into the corners. Fast counter-attacks must be effective, and if worse comes to worse, Micheal Owen might have a role to play.
Although Daniel Alves is much more attacking than his counterpart on the left wing, defending both flanks will be very important if United are to end the night victorious. David Villa and Pedro aren't natural wingers, and drift in from the flanks to participate in the "carousel" that produces chances for whoever runs on from the midfield. They do sometimes, however, wait on the wings for a "inside/out" ball from Xavi or Iniesta to run onto, from which they deliver an assist for an on-rushing midfielder or shoot themselves. The way they play also allows the right and left wing back the space and time to overlap and provide width which drags defenders out of position, and provide the "angled passes" which catches the defense out of position.
That said, playing such a high defensive line, and allowing Alves and the left back to get forward allows our own wingers and wing backs to get forward to provide offensive width and support counter attacks.