Have Europe's elite worked out how to beat Barça?

Messi and Co are almost invincible in La Liga, but shock Milan defeat was more evidence disciplined defence can stop their passing game – and that they lack a Plan B

Lionel Messi endured a frustrating night against Milan (EPA)

"We knew exactly what we were going to face," Sulley Muntari said afterwards, and this was precisely the point. Milan had just beaten Barcelona 2-0, leaving their chances of a third Champions League in five years in severe jeopardy.

The tie is not over and Barcelona have been beaten before. But they have never looked as impotent, unimaginative and stoppable as they did at San Siro on Wednesday evening. Milan knew how to defeat Barcelona – defending narrowly and as a unit – and the players stuck to it perfectly. Barça did not create a single good chance.

Twelve points clear at the top of La Liga having lost just once all season, this is not a regular occurrence. But it no longer feels like a tear in the fabric of football for them to be beaten, not by another European side with a smart manager and capable players.

Muntari, who scored the crucial second goal, said that Milan coach Massimiliano Allegri knew just what was needed to win the game. "We played as team and we listed to our coach," said the Ghanaian midfielder. "He did a really fantastic job putting us really well on the pitch. We were very disciplined on the pitch, we managed to close every angle that Barcelona has, so they couldn't create."

Knowing that Barcelona play entirely through the middle, that is where Allegri arranged his players. Milan employed a tight 4-5-1 formation, using a deep and narrow back four with as little space as possible between them. In front of them Riccardo Montolivo and Massimo Ambrosini were ferociously disciplined, closing down space and trying to disrupt Barcelona's intricate trigonometry. Muntari was the third central midfielder, slightly more advanced, sabotaging the Spanish side's attacks at the source.

In the first half Milan were happy to let Barcelona bring the ball into their own half, so keen were they not to concede an early goal. At the start of the second half, with Barcelona's rhythm gone, Milan started to press them higher up, trying to win the ball in their half instead. That approach brought their two goals.

Before half-time, Milan's two wide midfielders Stephan El Shaarawy and Kevin-Prince Boateng worked desperately hard following Barcelona's full-backs Dani Alves and Jordi Alba back into their own half. In the second half they pinned them back as Milan surged forward. "We played the game that we prepared," Montolivo explained, "closing down the spaces and then countering and making ourselves dangerous."

A plan like this has worked before. In 2010 Jose Mourinho's Internazionale beat Barcelona 3-1 at San Siro with a bold pressing display – like Milan's second half performance – before dogged defence – like Milan's first half – in the Nou Camp limited Barcelona to a 1-0 victory which sent Inter through to the final, which they won. Last year Roberto Di Matteo's Chelsea, an experienced team but not a great one, did something similar. They beat Barça 1-0 at Stamford Bridge before holding them to a 2-2 draw in Barcelona.

But those two victories, built on similar narrow defences, space-constriction and robotic discipline, did owe something to luck. Barcelona had a goal wrongly disallowed against Inter, which would have sent them through on away goals. And they were strikingly profligate against Chelsea, Lionel Messi missing a second-leg penalty.

Wednesday night, though, was slightly different. Barcelona did not create anything, looking uncharacteristically short of ideas. There was only one plan, to unpick Milan's defence with quick intricate passing, but it did not happen and there was never an alternative. No one would say that Barcelona would be better if only they were more like Stoke City but at times they do lack pace, physicality or a direct approach.

Pep Guardiola had a habit of making the required changes to brighten up his side but Jordi Roura, taking the team while Tito Vilanova receives cancer treatment, made just one tactical change, and that after 62 minutes and with his team already behind.

With Cesc Fabregas in midfield Andres Iniesta was pushed out onto the left of the front three where he did not make his usual impact. Playing the best midfielder of his generation on the wing feels like a damaging compromise, but when Alexis Sanchez came on to play there Barcelona were little better.

There is a sense that this Barcelona team is not as well balanced as when they had Samuel Eto'o and Thierry Henry in the front three, as they did in 2009, or David Villa cutting in from the left with Pedro on the outside, as they did in 2011. With so many midfielders, not enough width and not enough forward pace this team seems easier to predict and easier to stop than they have been before. Everything ends and without some changes at Barcelona this era of dominance may not have long left.

Schalke lodge Drogba protest

Schalke have lodged a complaint with Uefa over the 1-1 draw at Galatasaray in the Champions League because of Didier Drogba's participation. "There are doubts over the valid eligibility for Drogba," the club said. Fifa provisionally cleared Drogba to play last week after he moved from China's Shanghai Shenhua.