The previous two matches between AC Milan and Barcelona were brilliant games, providing 9 goals and much of tactical interest.

The previous two matches between AC Milan and Barcelona were brilliant games, providing 9 goals and much of tactical interest. Arguably, with the exception of perhaps Real Madrid, AC Milan are the toughest draw Barcelona could have got, which accounts for Guardiola's insistence on Barcelona's need to perform at the top of their game. After a draw at Camp Nou, it was only an exquisite pass from Lionel Messi which separated them at San Siro. The key tactical battle will unsurprisingly take place in midfield. The natural formation of each team creates a 4 v 4, with neither team having any natural width in that area. Milan's three man midfield can sit deep and prevent Barcelona from playing in between the lines, and also make it hard for Messi to drop deep and find space in front of the defence. They did this with reasonable success at Camp Nou, making a significant number of interceptions in that area (see diagram below), although they allowed Barcelona too many shots on goal.

Milan's trequartista (Kevin Prince Boateng) can also occupy Barcelona's defensive midfielder, meaning that he will no longer be as much of an option for retaining possession. Kevin Prince Boateng being available and playing in this position  should make a significant difference, as he is a high energy player and superb at pressing. Tracking midfield runners will be key: with the midfields likely to match each other movement will be important. This might cause Allegri to opt for Nocerino ahead of Aquilani, despite the later's passing ability, as Aquilani was poor at this during the San Siro game, failing to pick up Keita for Barca's first goal, and then fouling Xavi in the box for their second. It should also be noted that the winning goal in that game was scored by Xavi running into the box, highlighting the importance of tracking midfield runs.

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Barcelona are the more tactically flexible team and it seems likely that Guardiola will try to create an extra man in midfield. There are two ways in which he might try and do this, either by (1) dropping Busquets back, or by (2) pushing Dani Alves up.

(1) In the San Siro game, Guardiola did what he's done a few times this season and played with 3 at the back with Busquets just in front, dropping back to create a back 4 when necessary. While this usually means Busquets has more time on the ball to build attacking moves, in this game Boateng played higher up the pitch in order to push Busquets back, meaning Barca effectively ended up playing a back 4. It seems unlikely that Guardiola will take this option for the following reasons:

a) With Boateng being one of the players most likely to hurt Barcelona, Busquets job will be mainly dealing with him rather than building attacks. The aforementioned pressing ability of Boateng will also mean Busquets has less time on the ball.

b) A centre-back pairing of Pique and Puyol is probably best equipped to deal with Ibrahimovic (more on him latter). When Busquets has played in that auxiliary centre-back role, Puyol has usually been a defensive full-back. With the ariel threat and strength of Ibrahimovic, Gaurdiola will probably want Puyol in a central position.

(2) The attacking ability of Dani Alves needs no introduction, and he will naturally play very high up the pitch. Milan's narrow 4-3-1-2 formation leaves the opposition full backs unattended, and Alves will have license to push up and create a 5 v 4 in midfield. In the Camp Nou game, Milan coped reasonably well with Alves, forcing him inside most of the time, meaning that Barcelona were very narrow. However, this aslo meant that Pedro was allowed to essentially roam free, with the result that he was not marked for the first Barca goal. This seems to be the option Gaurdiola will most likely go for.

To conclude in regards to creating an extra-man in midfield, pushing Dani Alves forward seems the most sensible option for Guardiola playing against Milan's 4-3-1-2. Guardiola will most likely opt for a nominal back four rather than a back three with Busquets dropping back. While Alves' forward movement will essentially create a back 3, with Busquets perhaps dropping back as need dictates, the ostensible starting formation will most likely be more akin to the 4-3-3 at Camp Nou than the 3-4-3 at the San Siro.

The use of Alves raises another factor which will most likely be significant in the match: the use of width. In both teams' systems, width is provided primarily by the full-backs, and secondarily by other players drifting out into wide areas. It is therefore likely that there will be a significant amount of attacking play from the fullbacks on top of that already mentioned regarding Alves. Width played a significant role in the first-half of the San Siro game, with Barca's first goal coming from an Abidal surge, and Milan's from a wide run from Seedorf (again, note the midfield runner) which dragged Puyol away from Ibrahimovic.

The forward runs of the full-backs will give other players opportunity to drift into the space left behind them. Particularly likely to exploit this space is Robinho, who likes to drift wide (particularly left, see heatmap below) to pick up the ball. With both teams narrow and matching each other in midfield, the result could depend on which team best uses the ball in wider areas. This is also another reason why using Alves as the 5th man in Barcelona's midfield makes sense.

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Other points worth noting are Milan's injury problems, pace, and Zlatan Ibrahimovic. Milan are without Thiago Silva due to injury, which means Nesta will replace him at centre-back. While he has not played regularly or recently this season, he was fantastic in the Camp Nou game, so it remains to be seen how much of a loss Silva is. Barcelona's back-line are not the quickest, and struggled with the pace of Pato. Milan therefore have the option of both Pato and Emanuelson with which to try and trouble Barcelona. 

Also, Ibrahimovic is available, which he wasn't for the Camp Nou game, and is arguably Milan's most important player, providing goals and assists.  As he proved in the 4-0 win against Arsenal, he is fantastic at holding the ball up, as indeed he was against Barca at the San Siro (see diagram below). Ibrahimovic therefore gives Milan the option of bypassing the congested midfield and Barcelona's aggressive pressing high-up the field by playing direct passes to him. His height and strength suggest that Guardiola will want to match Pique or Puyol against him, rather than Mascherano.

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Overall, Barcelona will certainly be favourites going into this match: their ability to keep the ball as well as the individual genius of Messi et al. means they undoubtedly have the edge. Nevertheless, Milan pose a real threat to Barcelona, and are certainly capable of frustrating and even beating them. The use of width by each team with be key to the outcome.