Spurs' victory earlier this season was their first since 1989 at Old Trafford.  The match clearly exposed the failings of Ferguson's greatest tactical initiative since the exit of previous assistant manager Carlos Quiroz; Kagawa playing behind Robin Van Persie in a 4-2-3-1.

While United have by no means had a bad start (12 points from 6 games, 3rd in the league), the 4-2-3-1 has largely failed; they've lost to Everton and Spurs, been 2-1 down at St Mary's, and 1-0 down at Anfield (a match that thoroughly exposed United until Shelvey's sending off) and have scraped past Fulham 3-2 while using the system.

Their only league game of the season unmentioned was a convincing 4-0 victory versus Wigan, when they lined up in their traditional 4-4-2.

If we regard these stats properly, it's clear the system is inferior to the 4-4-2 played for the last couple of seasons. The squad, if anything, is stronger than before with Van Persie up front, so tactically something is wrong.

The system exposes the underlying problems in United's midfield

Scholes and Carrick by all means are consistent central midfielders, particularly at controlling games with their technical passing and defending with their intelligent positioning. Ultimately though, neither are especially athletic, and the midfield lacks energy.  The problem lies with Kagawa, who through little fault of his own, plays a detached role between the lines drifting out to the left flank, offering little support to both players.  This, consequently means  (a) Scholes and Carrick are outnumbered by any team who choose to play 3 central midfielders (Spurs did, as well as a majority of both teams) and (b) the team lack a designated destroyer to relieve pressure when without the ball.

United do have the players to change this in order to bring some variety into the team, but Ferguson seems uneasy at using them.  The ideal player would be to bring back Fletcher, but his fitness remains an issue, and with Kagawa already providing a very advanced option in the centre of pitch, its not surprising he does not opt for Anderson and Cleverley who like to drift forward.

What's more, the 4-2-3-1 emphasises Scholes and Carrick's exposure because United's wide players have more of an attacking mindset, as oppose to in a flat-ish 4-4-2 where they have some responsibility to track back. This means both players sit with little protection from opposition wide players.  Spurs clearly exploited this with both of their opening goals coming from left sided players (Vertongan and Bale), who got the better of Nani and Rafael who had pushed up aggressively.

Giggs

The flaws of the system are compounded somewhat more when Ryan Giggs is used on the left.  Giggs in his old age now offers similar attributes to the team as Scholes and Carrick- consistent passing ability but a lack of energy.  It looked as if Ferguson had acknowledged his midfield weakness, and therefore used Giggs to support that zone, but by doing so sacrificed any attacking threat on the left flank. Giggs clearly lacks the athleticism to do both roles- both support the midfield and attack the left flank; against Spurs he only received the ball twice in the final third, and at the same time only completed 5 passes- clearly offering nothing in both areas. United subsequently attacked solely through Nani on the right flank, becoming one dimensional and predictable.  Giggs was useless, and was rightfully withdrawn at half time.

Kagawa and Van Persie are isolated

With such weaknesses in midfield, it's no wonder that United have been slightly out of their depth.  Andre Villas-Boas and Brendan Rodgers both pressed Scholes and Carrick heavily, reducing their control on the game, meaning Kagawa and Van Persie became isolated.  Against Liverpool, prior to Shelvey's sending off, Kagawa received the ball only 12 times,  not one being in the centre of the pitch or in the final third- and Van Persie only got the ball in the Liverpool box once.

So why is the 4-4-2 more effective?

There isn't any obvious logic as to why the 4-4-2 is more successful- United maintain the same personnel in midfield. That said, there are some clear tactical alterations:-

Firstly, Ferguson has always demanded that both of his front men drop deep to mix play up- Welbeck and Hernandez both made themselves get into the game more to receive passes from Scholes and Carrick.  This is in contrast to the 4-2-3-1 where Kagawa is the designated number 10 and Van Persie the 9, which results in play being more predictable, static, and easier to defend against- especially for opposition holding players.

Second, Giggs and Nani both took up defined narrow wing roles which they looked more familiar with.  For one, both players acknowledged a responsibility to track back on Wigan's wide players- Boyce and Beausejour (thus helping Scholes and Carrick defensively), and second added attacking balance- the team exploited both sides of the pitch.

Finally, the change of strategy out wide allowed for United's attacking full backs to get further forward, safe in the knowledge that Giggs and Nani did acknowlege important defensive responsibilities and would cover for them if necessary.  It was no surprise that in the Wigan game Buttner was able to bomb completely forward, overload the flank and score an impressive debut goal inside the 6 yard box.

Final Thoughts

It would of course be rather simplistic to suggest that United's lackluster performances have been solely down to tactics- there are a range of other factors to also consider.

For one, Kagawa in himself is a highly unique Ferguson signing who the team may still have trouble adapting too given how they've consistently played over the last couple of decades.  It's difficult to remember Ferguson investing in a complete no.10 at any point in the past.

Furthermore, the team has been compounded with injuries, particularly in defence, which has probably brought about some inconsistencies.

It's only October and the season is still young- it could be very likely that the team will begin to gel very soon and become more consistent.  In the meantime however, Ferguson may just be tempted to revert back to his comfort zone and put faith in the 4-4-2 that has won him so much over the years.