Holders AC Milan will face Benfica and former midfielder Rui Costa on Tuesday when this season's Champions League group stages get underway on Tuesday.

- UEFA Champions League

The seven-times winners are without the injured Ronaldo [Images] and 39-year-old Paolo Maldini while Filippo Inzaghi, who scored both goals in the 2-1 victory over Liverpool in last season's final, is not assured of a start in the Group D clash at San Siro.

Liverpool, the 2005 champions, also face Portuguese opposition with a tricky trip to 2004 winners Porto.

John Arne Riise and midfielder Mohamed Sissoko have been ruled out of the Champions League Group A opener in Porto on Tuesday.

The left-sided Norwegian injured his groin on international duty last week and will not travel to Portugal, the club's Web site said on Monday. Sissoko is out with a leg injury.

Better news for last year's Champions League runners-up is the return to fitness of Fabio Aurelio, who will be in Rafael Benitez's squad to face the Portuguese champions.

Brazilian Aurelio has been out since April after rupturing his Achilles tendon against PSV Eindhoven in last season's Champions League quarter-final.

"Fabio is back which is good news for us," manager Rafael Benitez said. "He has worked hard to recover from his injury and could be a useful asset for us."

Also in Group A, Olympique Marseille return to the competition for the first time since 2003 when they host Besiktas.

Netherlands winger Arjen Robben could make his debut for Spanish champions Real Madrid at home to Werder Bremen with Lazio travelling to Olympiakos in the other Group C match.

"Yesterday we spoke with Robben, and it is probable that he will play his first minutes tomorrow," Schuster told a news conference on Monday.

"He's in good shape, and very enthusiastic, as are we that he can join the group and enter games as soon as possible."

Robben joined Real from Chelsea for 36 million euros ($49.92 million) at the end of August but has yet to make his first appearance for the club after working to shake off the effects of knee surgery from earlier this year.

Chelsea, semi-finalists last term, will be without striker Didier Drogba and midfielder Frank Lampard [Images] at home to Rosenborg Trondheim while Schalke 04 welcome Valencia in Group B.

Drogba misses out with a knee problem while Lampard is still troubled by the thigh injury that sidelined him for England's [Images] Euro 2008 [Images] qualifiers with Israel and Russia [Images] this month.

Chelsea coach Jose Mourinho told a news conference on Monday that Peru striker Claudio Pizarro is also doubtful with a calf strain.

Wayne Bridge has an ankle injury while centre-half Ricardo Carvalho (thigh) is also sidelined.

Mourinho said: "If you are talking about Drogba, Lampard, (Michael) Ballack and Carvalho you are talking about 40 percent of the team, the spine of the team.

"Wayne Bridge is also out leaving Ashley Cole as the only left-back to play every minute of every match."

Germany [Images] skipper Ballack, not named in Chelsea's 25-man Champions League squad, would not have played even if he was eligible as he still has not recovered from his ankle injury.

Scottish champions Celtic visit cash-rich Ukrainians Shakhtar Donetsk in the other Group D match with the fixtures in Groups E-H taking place on Wednesday.