Milan put in a solid defensive performance to frustrate a narrow Barcelona.

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In terms of overall shape, Barcelona lined up (as expected) with a nominal back 4, Busquets, Xavi and Keita in midfield and a front three of Iniesta, Messi and Sanchez. Messi often dropped deep and to the left to find space around Milan's midfield, adding an extra man for Barca in that area when he did so. Iniesta, who is not a natural wide player in front 3, tucked inside and essentially created a 4 v 4 in midfield. Keita occasionally drifted wide to allow Iniesta to cut inside, but Alves was the only real wide player for Barcelona. While Barcelona lined up in a 4-3-3, when they were in possession it became a 3-4-3, as Alves pushed high up the pitch, or even a 3-5-2 when Messi dropped deep or Iniesta cut in. There was a huge difference in the roles of Alves at right-back and Puyol at left-back, with the latter essentially a third centre-back and the former (as expected) playing in midfield when Barcelona were in possession (see diagram below).

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The following screenshots clearly show how Barca effectively played a back three with Alves (blue dot) further forward.

AC Milan 0 2013 0 Barcelona | FootyRoom - Latest Football Highlights

AC Milan 0 2013 0 Barcelona | FootyRoom - Latest Football Highlights

Milan lined up in their normal 4-3-1-2. Due to injuries the back 4 was not Milan's first choice, and Ambrosini replaced Van Bommel at the base of their diamond. Robinho was also fit and played up from with Ibrahimovic.

Milan pressed hard in the first 20 minutes, and an interception from Seedorf in the 3rd min resulted in a chance that Robinho should've scored. Again in the 19th min Xavi was caught in possession and Ibrahimovic had a chance which he should've put away. The benefit of Barca's shape was that Alves could push up and create width, but the downside was that when Milan pressed in numbers they were caught with Alves out of position. The screenshot below shows how Seedorf's 3rd min interception created a 4 v 3 situation for Milan.
AC Milan 0 2013 0 Barcelona | FootyRoom - Latest Football Highlights
Milan really should've capitalised on these early chances; they were brilliant chances, and such opportunities are often limited against Barcelona.
Milan pressed with their front two lines (Ibrahimovic, Robinho and Boateng) when Barca had the ball at the back. Once the ball was played behind these front two lines, the defence and midfield retreat and sat in front of their penalty area, making it hard for Barca to play in between the lines and break them down. Milan we happy for Xavi to spray the ball around in front of the defence, and frustrated Barcelona with a decent defensive performance, as the diagram below shows.
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Although Barca did manage to play through Milan's defence a few times (e.g. Xavi's shottheir chances were limited. Barcelona really need width to their play in order to stretch the compact Milan defence. Alves was the only player who really gave any width to Barcelona's play, and was often in space on the right hand side (see below, blue dot). However, as the diagram below shows he often cut inside and passed back into the centre, rather than putting crosses in (understandable, with Barca forwards lacking in height compared to Milan's defence). It's notable that the only time Barca put the ball in the net, it was after the ball was played to Alves, but Messi was ruled offside.
AC Milan 0 2013 0 Barcelona | FootyRoom - Latest Football Highlights
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Guardiola didn't adress Barcelona's lack of width until Tello came on for Iniesta in the second half. He is a much more natural wide player than Iniesta, and really helped to stretch the play. He was often found in space on the left (see below, blue dot) and created a few chances.
AC Milan 0 2013 0 Barcelona | FootyRoom - Latest Football Highlights
Although 0-0 is a reasonably good result for Milan, as a score is enough for them at Camp Nou. However, they should be disappointed not to go into that game with a goal. Other than the chances they missed early on, they could've perhaps done more to hurt Milan. Robinho was particularly disappointing: as the forward operating largely on the left it was his job to exploit the space in behind Alves, which he didn't do particularly well. Even when he came off for El Shaarway Milan didn't really exploit that space on their left, which they will need to do.
Milan will be happy not to have conceded, but should really have scored in this game. Barcelona will need more width to overcome the Milan defence in the second leg.