The Serie A side top Group C, on the same points as Real Madrid, but the French outfit are one point back with two games to play.
Milan's season, less than four months old, has had more twists and turns than a mazy run from Ronaldinho back in his prime at Barcelona.
A rollercoaster start to the campaign, hallmarked by a shock home defeat by FC Zurich and a win at Real, has been followed by a steady improvement while the Brazilian has found consistency after a frustrating first 16 months in Italy.
The 29-year-old, now settled on the left of a front three, blasted in a penalty and produced some exquisite touches in Sunday's 4-3 win over Cagliari.
"For me it's easy to be able to play with intelligent players like Marco Borriello and Alexandre Pato who understand what I can do for them on the pitch," Ronaldinho told reporters.
"Right now playing at the San Siro makes me feel good, here I feel at home, the fans understand I needed their support and this makes me happy."
Forward Borriello was so impressive against Cagliari he could keep European specialist Filippo Inzaghi out of the lineup while coach Leonardo will have defender Alessandro Nesta back from suspension after shipping three bad goals on Sunday.
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Marseille boosted morale with a 1-0 victory over arch-rivals Paris St Germain on Friday.
The former European champions, four points behind surprise leaders Auxerre in Ligue 1, have a doubt over midfielder Stephane Mbia (ankle sprain).
"It's nice to have beaten Paris before going to Milan, it's good for our confidence," said Argentina defender Gabriel Heinze.
"Milan are a different proposition but I believe we can win there. We have to if we want to qualify," added Heinze who scored the winner against PSG.
Midfielder Fabrice Abriel said the man to watch would be Clarence Seedorf, who set up both Inzaghi's goals in Milan's 2-1 win at Marseille in September.
"Seedorf is their most important player and we'll have to keep an eye on him," said Abriel. "He's the one who gives all the good balls, he's very intelligent and he's real added value for Milan."
If Marseille lose, Real will go through at their expense with a home win over Zurich.
Probable teams:
Milan: 1-Dida; 44-Massimo Oddo, 33-Thiago Silva, 13-Alessandro Nesta, 15-Gianluca Zambrotta; 23-Massimo Ambrosini, 10-Clarence Seedorf, 21-Andrea Pirlo; 7-Alexandre Pato, 22-Marco Borriello, 80-Ronaldinho
Marseille: 30-Steve Mandanda; 24-Laurent Bonnart, 21-Souleymane Diawara, 19-Gabriel Heinze, 3-Taye Taiwo; 17-Stephane Mbia (or Edouard Cisse), 7-Benoit Cheyrou, 18-Fabrice Abriel, 8-Lucho Gonzalez; 9-Brandao, 11-Mamadou Niang.
Referee: Howard Webb (England)
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