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In recent seasons, Manchester United manager Sir Alex Ferguson has often elected to use a 4-3-3/4-5-1 hybrid shape for his side in so-called "big" matches. Domestically, this generally has meant clashes with Chelsea FC, Liverpool FC, Arsenal FC, and more recently, Manchester City. However, the genesis of this tactic for the United boss came about when Fergie realized that an evolution of his tactics were needed if United were to succeed in European competition. These tactics then became suitable on the domestic front when these same English opponents adjusted their own tactics, due to their own experiences in Europe and/or because of the influence of foreign managers.
Over the past 10-15 years, in places like Spain, France, Portugal, Italy, and the Netherlands, the dominant use of shape has changed from a 4-4-2 (or variations of it) to shapes that include three-man central midfields; mainly 4-3-3, 4-2-3-1, or 4-3-2-1. The basic advantage that a three-man central midfield shape provides over a generic 4-4-2 shape is a '3 v 2' advantage in the center of the pitch. Theoretically if all things are roughly equal, the side with three central midfielders will control possession by having a numerical advantage in this area of the pitch where possession is most influenced by. The three can simply pass around their two counterparts.
This basic diagram depicts a 4-4-2 (red) versus a 4-3-3 (black). As you can see in the yellow box, the black side has a '3 v 2' advantage in the center of the pitch:

So, the obvious question might then be "if the three-man central midfield side have a numerical advantage in the center of the pitch, they must be deficient elsewhere on the pitch?" If a 4-4-2 side comes up against three-man central midfield, the basic strategy will be to try to swing the ball to the wide areas and play with width. Get the ball out of the zone where you have a numerical disadvantage. Your wingers and strikers should all have one-on-one match-ups elsewhere on the pitch. However, this is easier said than done. If the three-man central midfield is controlling the possession in the center of the pitch, then the 4-4-2 may be starved of opportunities to take advantage in the areas that they have might have an edge. Therefore, in basic theory, the 4-4-2 side can only outplay their three-man central midfield opponents if their two central midfielders manage a way to out wit their three counterparts; this is possible through sheer effort and/or superior technical ability.
Another question one might pose, "does the three-man midfield have a disadvantage with a single striker?" My answer: Not really if (1) you're controlling possession in the center of the pitch and if (2) you have fluid movement in attack. One thing a 4-3-3 gives you is passing triangles all throughout the pitch; this makes it easier to control possession with short passes. This diagram below displays that in a basic 4-3-3 shape:
If the movement in attack is smart and fluid, then this opens up a number of possibilities. It would appear the single striker is at a disadvantage due to having two-center-backs marking him. However, this lone striker can use this to his advantage by coming deep and dragging a center-back out of position, thus, he creates space in behind him for the wide players or midfielders to run onto. The extra center-back is then forced to cover too much space for these variety of midfield runners. The lone striker can also move into the wide areas, again possibly dragging a center-back out of position. If the wide attackers are reading the situation right, they can interchange smartly and expose any open space a defender might allow in these fluid movements. So, the main point is, the lone striker can actually expose two center-backs at once if his movements are good. Recent high profile examples include Carlos Tevez for City, Wayne Rooney (2009-10), and Lionel Messi (2010-current) for Barcelona.
Again, this is all general stuff when it comes to these shapes and my description of the 4-4-2's theoretical weakness against three-man central midfield systems is only to highlight the reasoning why Fergie has often switched to a 4-3-3. It certainly has its merits. However, it is important to point out that United do not play in a specific shape. The Gaffer has used a variety of shapes in recent seasons: 4-4-2, 4-2-3-1, 4-3-3, among others.
UNITED'S PAST USAGE OF 4-3-3/4-5-1
Michael Cox, the editor of Zonal Marking, does an excellent job at discussing the tactics used by United in 2006-09, seasons in which they won three straight Premier League titles and appeared in two Champions League finals. Here are few highlights from Cox's piece:
* When Fergie used a 4-3-3/4-5-1 during this stretch, he had highly versatile attacking players in Rooney, Tevez, and Cristiano Ronaldo. All three could effectively play on either flank or as the frontman.
* Depending on match-ups, Fergie could also deploy Park Ji-Sung on either flank as a defensive attacker. The Korean could also play in the central midfield if his energy and defensive abilities were needed there.
* Paul Scholes, Michael Carrick, Darren Fletcher, and Owen Hargreaves were all flexible as central midfielders and could be featured in a variety of roles; whether that be in a 4-4-2 or a 4-3-3.
* A big club like United requires squad rotation because of the volume of games they play each season. Because of this, versatile players are needed to fit in a number of systems.
Just to expand a bit on Cox's points, the key to the versatility of Rooney, Tevez, and Ronaldo is that it enabled Fergie to make slight adjustments in matches. Ronaldo wasn't, and still isn't, the best at tracking back to defend against full-backs that get forward. If United were up against two of these attacking full-backs, he had the hard-working Rooney and Tevez that he could put on the flanks to track them. If Fergie saw a defensive liability from an opponent on the flank, he could put his best attacker (Ronaldo) there to expose him. In basic terms, if something wasn't working for United, they didn't have to make drastic shifts to their shape as they could use the versatility of this trio to find the right match-ups versus various opponents.
That 2007-08 side were spectacular to watch and were fascinating tactically as well. They were also very well suited to playing in a 4-3-3. So much so that they caused this writer to skip class too often during the Champions League run and flunk a class.
UNITED'S CURRENT USAGE OF 4-3-3/4-5-1
By using the word "current," I mainly speak of the post-Ronaldo-and-Tevez era. During the 2009-10 season, Fergie continued to use the 4-3-3/4-5-1 shape often. This mainly consisted of Rooney as the fulcrum in attack, a role that saw him become a 30+ goalscorer. Antonio Valencia and later on, Nani, both emerged as influential players as well. The club also acquired Michael Owen and Dimitar Berbatov became featured on a more frequent basis. Park continued his role as a defensive attacker. Not much changed in the central midfield other than the unfortunate and recurring injuries to Hargreaves. In the current season, Javier 'Chicharito' Hernandez was brought in to enhance the attack.
To state the obvious, Ronaldo and Tevez are tremendous footballers and their losses were always going to be difficult to overcome. However, I don't think the decline in the effectiveness of United's 4-3-3/4-5-1 is due to a drastic drop in quality because Valencia and Nani have emerged as two of the best players in the Premier League. I'd attribute it to the lack of versatility in the current personnel and the dependence that his shape has on Rooney. To highlight on this latter point, United's season fell apart last Spring when Rooney became injured.
In a 4-3-3/4-5-1, United appear to only have one viable candidate to lead the line in attack; Rooney. They no longer have the versatility and quality of a Ronaldo or Tevez to provide an alternative in the case of a less than optimal match-up, injury, or loss of form. When the latter two issues popped up last Spring and through the early parts of this calendar year in regards to Rooney, United proved how stagnant they were in a 4-3-3/4-5-1 without him. When he did regain form in the past few months, it wasn't as the fulcrum in attack, it was in a withdrawn playmaking role. When Berbatov or Chicharito have been featured as the fulcrum in attack in a 4-3-3/4-5-1, both have failed to inspire. Although I haven't completely written off the Mexican striker due to his tremendous movement off the ball. Furthermore, Owen has shown throughout his entire career that he is not effective as a lone striker. Thus, the former England international is better suited to a role in a 4-4-2. This failure to find anyone else other than Rooney as an effective fulcrum in attack is the biggest reason why I think the 4-3-3/4-5-1 is no longer viable for the club at the present moment. This goes hand-in-hand with versatility.
The lack of versatility extends to a lesser degree in other areas of the pitch. Valencia is a terrific player but he can only play effectively on the right side. This limits options. Nani can dominate a match from either side, but he prefers, and is better, on the right side. Fergie simply doesn't have the subtle options in a match to make a change without having to make a drastic one. During the past four months, the 4-3-3/4-5-1 has been ineffective. I'd argue only the recent Fulham match is where it's been somewhat of use. If you look back through our match review archives, you will see how many times Fergie completely switched shape in the middle of a match when the 4-3-3/4-5-1 failed to yield desired results. We have the talent on this squad in attack, we simply don't have the versatility and personnel to make this particular shape consistently work.
CONCLUDING THOUGHTS ABOUT THE RUN-IN
Get rid of the 4-3-3/4-5-1 for the remainder of this season. Simple as that. As previously mentioned, the only viable option to lead the attacking line is Rooney, however, he has become our talisman again in a withdrawn playmaking role. This has occurred in our effective 4-4-2/4-2-3-1. With Wazza willing to drop so deep when United are out of possession, this has allowed him to get continually goal-side of the opponents deepest lying midfielder. Therefore, he prevents us from being overrun in the central midfield in a '3 v 2' situation. In addition, we have an in-form striker in Chicharito and the Prem's leading goal-scorer in Berbatov, who both seem to thrive with Rooney in this withdrawn role.
In this 4-4-2/4-2-3-1 shape, we once again gain flexibility. We have two terrific striker choices to pair with Rooney. We have the option of Valencia, Nani, or Park on the right and the option of Nani, Giggs, or Park on the left. In the center of the pitch, where United have arguably been the weakest this season, this only forces them to deploy two-central midfielders to play deeper than Rooney. Depending on tactics or match-ups, Carrick, Giggs, Scholes, Anderson, Fletcher, and Park all provide quality options there.
Furthermore, when examining the remaining fixture list, there aren't too many teams where we need to be concerned about being overrun by in the central midfield. We've already proven that our 4-4-2/4-2-3-1 is effective against Chelsea. In our Champions League semi-final tie, Schalke '04 will likely play in a 4-4-2/4-4-1-1 sort of shape. The two biggest concerns would be Arsenal in league play and a potential Champions League final versus Barcelona. However, with Rooney's terrific work-rate and willingness to come deep to defend, I contend that the tried-and-true 4-4-2 is the way to go for the run-in.