Marseille, third in Group C and three points behind leaders Real, need to win by at least three goals to advance into the last 16 if second-placed AC Milan beat FC Zurich.
"Beat Real 3-0? Everything is possible in football," Argentina midfielder Lucho told reporters after scoring a goal in Marseille's 3-1 win at Nice on Saturday.
Compatriot and Real Madrid striker Gonzalo Higuain said it was hard to imagine the Spanish giants going out after their 3-0 win over Marseille at Bernabeu in September.
"Something very improbable would have to happen for them to knock us out," he said.
Real have had moments when they have gone to sleep, like on Saturday, when they conceded two goals in four minutes against Almeria, although they won 4-2.
Marseille, however, have a lot of firepower in Senegal striker Mamadou Niang, Brazilian Brandao and Ivory Coast's Bakari Kone, with Lucho and Fabrice Abriel providing the perfect launchpad.
Didier Deschamps, who knocked Real out in the quarter-finals in 2003/04 when he was coaching Monaco, was not as optimistic as Lucho about his team's prospects.
"I did not like our defence (against Nice) on the through balls. There were too many hesitations and too many lost challenges," he said.
"However, this gave us confidence before the Real game. But we will have to be better on Tuesday.
RONALDO RETURNS
"Everybody will have to be at 100 percent and hope that Real won't be. The goal is to win Tuesday's game and we don't know what will happen in Zurich."
Should Milan slip up at Zurich, Marseille will not need to fire at least three goals past Real to progress.
They will also be boosted by the absence of influential playmaker Kaka from the Real line-up. He has been left out due to a nagging groin problem.
But Real coach Manuel Pellegrini was confident his team will not crack at the Stade Velodrome.
"Against Almeria we didn't lose our heads when they took the lead and we patiently and successfully turned the game round," Pellegrini said.
Portugal winger Cristiano Ronaldo's return to action following an ankle injury will bolster the attack.
Ronaldo set up the first against Almeria, won the penalty for Real's third and scored the fourth as they cemented second place in La Liga.
French striker Karim Benzema, who finally discovered his touch to score on Saturday, was looking forward to returning to his home country.
"I am in a big club with a lot of pressure but I feel comfortable. It's a pleasure to be able to return to France and we are expecting a tough game in Marseille," Benzema told a news conference on Sunday.
Probable teams:
Marseille: 30-Steve Mandanda; 24-Laurent Bonnart, 21-Souleymane Diawara, 19-Gabriel Heinze, 3-Taye Taiwo; 6-Edouard
