For the second time in this UEFA Champions League season, Barcelona have been stunned by a team that wasn't supposed to stand a chance against them. After losing away at Celtic 2-1 on Nov. 7 in the group stage, Barca were upset 2-0 by AC Milan in the first leg of the Round of 16 on Wednesday.

This might have been a skillful contest between well-drilled sides, most resembling a game of chess - but it made for unpleasant watching. But for two brief moments of high drama, the game at the San Siro was as unsightly as Barca's away jerseys - a reverse Tequila Sunrise of orange bleeding painfully into a clashing yellow.

More to the point, Barcelona now have some ugly questions to answer. They were without their coach Wednesday night - Tito Vilanova is recovering from cancer surgery in New York - and looked unprepared for Milan's rigorous defense. It was the tenth straight game in which the Catalan giants had conceded a goal. And for the first time in fourteen games, their magician, Lionel Messi, failed to score. As it stands, they now must score at least three goals at Nou Camp in three weeks time - without conceding any more.

There was little joy to be had from a game in which Milan packed in behind the ball, with as many as 10 players cramming into their own box when Barcelona threatened to advance too far. But Barca, for their part, went long stretches where they had all of the ball but seemed to have little idea of what to do with it. Feckless and sluggish, Barca never pelted Milan with the onslaught of quick, short passes and diagonal through balls they typically employ to break down opponents resigned to absorbing pressure. This was as much the failing of the Catalans as it was a credit to the Italians, whose organization and game-plan proved structurally sound.

Barca's malpractice in the final third gave Milan the opportunity to prey on the break-away. That's how Stephan El Shaarawy - who continues to be a revelation - was sprung in the 16th minute. The 20-year-old Egyptian-Italian forward took a heavy touch, however, allowing Carles Puyol to recover and punt the ball behind for a corner. On the ensuing corner, Kevin-Prince Boateng smacked the low ball played to him just wide of the far post. It was the only notable action of the half.

In the second half, El Shaarawy continued to go on long runs, utilizing his speed to drop behind Barca right back Daniel Alves and get Puyol and his central partner Gerard Pique into trouble. He lacked support, however, and the stalemate was retained.

Then, in the 56th minute the game finally broke open. Riccardo Montolivo struck an indirect Milan free kick from 25 yards or so into the obviously raised arms of teammate Cristian Zapata, posted by the edge of the box. The ball fell tidily into the feet of Boateng, who swept his left-footed shot past Victor Valdes. To the rightful consternation of the Barca players, the goal was allowed to stand.

Barca's game acquired some urgency after this lamentable non-call, but continued to lack venom. Andres Iniesta's shot sizzled just wide of goal in the 76th minute, while a Xavi Hernandez free kick swerved a foot or so high four minutes later. Uncharacteristically, those were all the chances Barca would muster all night, inviting further punishment for their haplessness.

It duly came in the 81st minute, when Milan's M'Baye Niang was found in the box, played the ball square for El Shaarawy, who chipped it on to the on-rushing Sulley Muntari, whose volley Valdes had no chance to repel, making it 2-0.

Milan's calculating performance, to borrow a somewhat hackneyed soccer stereotype, was typically Italian. Yet it should not have come as a surprise to Barcelona, as it was much the same way in which the Rossoneri got two results against them last year, when they twice earned a draw, in spite of being badly outplayed, by sitting deep and counter-punching.

As the clock ticked down on the first leg, the mix of ebullience and shock at the San Siro was best embodied by the club's new striker Mario Balotelli, who was ineligible to play because he'd appeared in the Champions League for Manchester City earlier this season. A camera caught Balotelli and some friends hopping and bouncing about deliriously , forming an impromptu mosh pit of sorts.

It made for a fitting image, given how shaken up Barca looked walking off the field after the final whistle.

In Istanbul, Galatasaray and Schalke ended up deadlocked 1-1 after a wide-open thriller that saw both teams squander huge chances. New boys Didier Drogba and Wesley Sneijder both started for Galatasaray but neither man looked fully fit, and Sneijder was yanked at the half.

The Turks struck first behind red-hot Burak Yilmaz, latching on to a pass from Selcuk Inan, flipping the ball past a hapless Schalke back Benedikt Howedes and into the back of the net. American Jermaine Jones would level just before the break off a long release from Klaas-Jan Huntelaar that sent Jefferson Farfan skating through the Turks' back line. He squared, and Jones side-footed it home.

Both teams could have had more; Huntelaar fluffed what should have been an easy tap-in; Hamit Altintop followed on with a rocket off Timo Hildebrand's bar.

Schalke take a precious away goal back to Gelsenkirchen for their meeting in three weeks' time but both teams look like they can seize this tie.

FOX Soccer's Jamie Trecker contributed to this report.