- Milan win 2-0 in first leg of Last 16 tie to stun tournament favorites
- Ghanaian duo Kevin-Prince Boateng and Sulley Muntari fired home in second half
- Turks Galatasaray held 1-1 at home by Schalke despite fielding Sneijder and Drogba
Barcelona stand-in coach Jordi Roura suffered another agonizing display on his competitive return to the stadium that effectively ended his playing career.
Twenty-four years on from suffering a cruciate ligament injury while playing for Barcelona in the San Siro, that same old sinking feeling returned for a man holding the reins as regular coach Tito Vilanova undergoes medical treatment for a tumor in New York.
Fielding a strong side against a Milan team that failed to beat the Spaniards in last season's four European Champions League meetings, Barcelona lost the first leg of the last 16 clash 2-0.
After a tight first half, Kevin-Prince Boateng broke the deadlock after 56 minutes when drilling home from the edge of the box after a Riccardo Montolivo drive was deflected into his path.
Nine minutes from time, fellow Ghanaian Sulley Muntari sealed Milan's first Champions League win over Barca in 13 years after he stylishly finished off a sweeping move involving Stephan El Shaarawy and new signing M'Baye Niang.
The victory was especially satisfying for the Milan hierarchy and club president Silvio Berlusconi, who may hope the unexpected win represents a good omen as the 76-year-old tries to return to office in this weekend's Italian elections.
Following last year's comprehensive 3-1 aggregate defeat against Lionel Messi and colleagues in the Champions League quarterfinals, Milan overhauled their squad -- with Gennaro Gattuso, Clarence Seedorf, Alessandro Nesta and Zlatan Ibrahimovic among those moved on.
A total of eleven players who took part in the 2012 clashes were sold, reducing the average age of the squad from 30 to 27, and the team has slowly overcome a difficult bedding-in period to rise up to fourth in Serie A.
Milan coach Massimiliano Allegri had spoken of the need before the game to find a way of blunting Barcelona's attacking threat and his plan worked to perfection thanks to the on-field discipline of his players.
"We played as a team and we listened to our coach," said Muntari, whose goal was his first ever in the Champions League.
"He did a fantastic job by setting us up us very well on the pitch -- and we were very disciplined and managed to close every angle."
Even though Milan are seven-time European champions, their tally second only to the nine of Barcelona's great rivals Real Madrid, exuberant celebrations spread out across the stadium as the four-time winners fell without scoring an away goal.
Roura has never made any claims to wanting to be a permanent coach and he suffered in Milan where the home side's impressive defensive discipline smothered the usual attacking prowess of a side standing 12 points clear in the Spanish league.
His side were unable to formulate an alternative to their usual fluent style until the stand-in coach chose to use defender Gerard Pique in a makeshift attacking role for the final minutes.
Mustering barely a shot on target worthy of the name, Barcelona's limp display -- with Lionel Messi's impact wholly minimized by some efficient defending -- must have made Roura, 45, wish he could never return to the San Siro in a competitive capacity again.
In 1989, he was an aspiring Barca midfielder when the then 22-year-old suffered a serious knee against Milan in the European Super Cup - an injury that would bring his career to a premature end four years later.
His side are now in danger of failing to reach the Champions League semifinals for the first time in six years and Vilanova may discover that his team is out of the competition when he returns to the fray in late March.
Elsewhere, Galatasaray fielded both major January signings -- Wesley Sneijder and Didier Drogba -- in their home clash with Germans Schalke but were held to a disappointing 1-1 draw.
The hosts appeared to have made the perfect start when Burak Yilmaz slammed home a fine individual goal after 12 minutes, with the striker now having scored seven of Galatasaray's 8 Champions League goals this term.
That was as good as it got for the home side, with Jermaine Jones leveling the scores shortly before halftime as the visitors finished the game the stronger.