3 Keys to Barca comeback

Barca become the first team to overcome a 2-0 first leg away defeat in the Champions League knockout stages, advancing through to the quarterfinals with a 4-0 win over Milan at the Nou Camp.

A pair of first half goals from Lionel Messi evened the tie at 2-2 on aggregate before David Villa tucked home the winner in the 55th minute. Jordi Alba put the tie beyond doubt with a fourth in stoppage time. Here are three key factors that contributed to Barca's historic comeback.

1. David Villa replaces Fabregas in starting XI
Barcelona struggled to create meaningful scoring chances in the first leg defeat largely because they lacked a threat behind Milan's center back pairing of Philippe Mexes and Cristian Zapata. In that game, Messi was used in his normal roaming center forward position but mainly stayed just to the right of the center of the pitch in Mexes' zone. Fabregas was given a free role in the middle and mainly looked to find space in between the lines towards the left. Andres Iniesta was therefore used on the left wing.

There were two problems with this lineup. Iniesta likes to cut inside when he plays on the left. With Fabregas occupying a narrow left-sided position, the two often seemed to get in the way of one another when Iniesta cut in. The second problem also stemmed from the inclusion of Fabregas. He maintained a fairly deep position, never looking to make runs in behind the Milan center backs. As he always does when playing center forward, Messi continually looked to drop in the space between Milan's center backs and midfield to collect the ball between the lines. However, because Barca didn't have a player more advanced than Messi looking to make runs in behind, Mexes was able to stay tight on the back of Messi when he looked to drop between the lines to collect the ball, knowing Zapata was the spare man behind him and could provide cover for any runs that came from midfield. Messi was quiet and Barca only had one shot on target.

The decision to drop Fabregas and replace him with Villa last night meant a bit of lineup reshuffling and a change in tactics for Jordi Roura. Villa was employed at center forward. Messi played a very narrow right sided position, Iniesta switched from wide on the left to a central attacking midfield spot and Pedro switched from a wide right to a wide left position. Althought Villa was relatively quiet before providing the goal that would see Barca through, his inclusion as the most advanced attacker was vital in opening space for Messi to receive the ball between the lines. Villa offered a threat behind the Milan center backs and stayed on the shoulder of Mexes. Mexes could therefore no longer step forward between the lines to close down Messi. To do so would have meant leaving Villa free in the area or relying on Zapata to slide over to pick him up- which would have opened up acres of space on the right side of Milan's box. Messi sat just behind Montolivo on the left side of Milan's three man center midfield and in front of Mexes and had the space to collect the ball and run at defenders. His finishers were out of this world but Villa's inclusion had a lot to do with why he was so much more dangerous than at the San Siro.

2. Adriano replaces Pedro in the 84th
When David Villa tucked home Barca's third, Milan right back Ignazio Abate became much braver with his positioning as the Italian side were forced to chase a goal that would put them through. Between Villa's goal and the introduction of Adriano, Abate was time and again given the space to overlap down the right wing unmarked as Jordi Alba was occupied defending Kevin Prince Boateng in narrower areas. He sent in some decent low hit balls across the face of goal and one was nearly converted (by Boateng if I remember correctly). Barca were on the back foot, defending deeper and it looked for a time that Milan would get a number of chances in front of goal in the final minutes off of balls whipped in from Abate. However, Roura brought in Adriano to offer protection in front of Alba and track the forward runs of Abate. The Milan right back was unable to get in space on the wing after the change and didn't get a ball in the box for the remainder of the game.

3. Niang's miss
Who knows how the game would have played out had Niang put away the 1 v. 1 chance against Victor Valdes that struck the post but the psychological impact on both teams would have been massive had he leveled the score there. As it turned out Messi would even the tie at two just a minute later. Barca could smell blood and from there it seemed inevitable they'd finish the job.