Manchester United's tactical flexibility has been the most remarkable aspect of the team's performances this season. When necessary, Sir Alex Ferguson has ripped up the book and deployed a most un-United-like diamond formation with little width and a compact midfield.
On the plus side, the deployment of the diamond has allowed United to control the midfield by having enough bodies in the central positions. This has occasionally helped overcome the absence of a ball-winning midfielder in the team.
On the negative side though, United have lost width in this formation and have been susceptible to marauding runs by opposition wingers and fullbacks.
The negative side-effects of the diamond were amply illustrated by the midweek scare against SC Braga. Sir Alex would have noted that United's recovery was in large part due to a move to the 4-4-1-1/4-2-3-1 formation with two wingers and two central midfielders.
The first reason has to do with the way Chelsea use the 4-2-3-1 formation. It is expected that the likely lineup in Sunday's game will see John Obi Mikel partner Ramires in central midfield with the in-form trio of Eden Hazard, Juan Mata and Oscar in front of them. Fernando Torres will probably be alone in leading the line for the Blues.
By playing the diamond, Manchester United can assume control of the midfield by denying Mikel and Ramires the ball. Both these Chelsea players are athletic midfielders who like to go forward in attack, but they lack the ball-winning capability of ex-Chelsea players like Claude Makelele or Michael Essien. A narrow midfield comprising of Michael Carrick, Tom Cleverley and one of Shinji Kagawa or Anderson can use their passing skills to play around the Chelsea midfield.
The dominance of possession will have two consequences.
First, United would cut the supply lines to the Chelsea front four by preventing their opponents from getting the ball. Second, the lack of possession would force one or more of Mata, Oscar and Hazard to drop deeper to get the ball, which would likely blunt their attacking contribution.
Even if the Chelsea midfield or back line did succeed in getting the ball to the front four, the narrowness of the diamond would more often than not force them to go into the wide positions. This would play into United's hands since Chelsea's three attacking midfielders have done the most damage when drifting inside into central areas this season.
Forcing the likes of Hazard and Mata to go wide instead of cutting inside will probably blunt their attacking threat since Chelsea no longer retain the imposing physical presence of Didier Drogba up front to get at the end of crosses. In other words, if Chelsea are forced to use crosses from the wings to create their goalscoring chances, United should be able to deal with that threat more easily.
On the attacking side, playing the diamond with Wayne Rooney at the tip would give the central midfield of Ramires and Mikel a different challenge defensively. Rooney has been in sparkling form in this role and would pose the biggest defensive challenge of the season for Chelsea.
In addition, the shuttlers in the United diamond could link up with Rooney and the two forwards during the team's attacks to outnumber the two Chelsea midfielders.
Additionally, the presence of two strikers in Robin van Persie and Chicharito/Danny Welbeck would give the central defensive partnership of David Luiz and Gary Cahill their most difficult challenge yet. Positioning has never been Luiz's greatest strength—something that the likes of RvP and, especially, Chicharito can exploit.
To summarize, the most intriguing tactical question for this game is how United can stop Chelsea's attacking prowess while retaining their own offensive edge. The 4-4-1-1/4-2-3-1 surely cannot be the answer, since Chelsea could just replicate Tottenham's game plan at Old Trafford to pose United problems.
On the other hand, the diamond can help stifle Chelsea's threat through the middle while allowing United enough opportunities to create chances themselves. Sir Alex will be aware of this fact, so he must resist the temptation to tinker with his formation for the sake of surprise and stick with this formation for the upcoming match.