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The odds at home team's win are under 1,9 which would be a probability of over 50%. I don't see why Marseilles are such heavy favorites here? They won only two our of their 15 home games - in August to the worst team in the championship Troyes and Bastila in September. Bastila are very bad away. Marseille didn't win home for 6 months. Looking at away games, Olympique are a bit better since they recorded their last win in February. But, only luck saved them from defeat or a draw there.
Things at the club is far from great. The fans are allegedly very angry and the players seem to have lost confidence. Their Spanish coach is very close to getting sacked, while Diarra and Alessandrini aren't on the team. That says it all about them.
On the other hand, Rennes are getting better. They got 7 points in the last 3 games. Lyon were closer to winning but getting back from a two-point deficit against a team like that says it all about you. Courbis didn't start well at Rennes. but his team is getting better each next round. They are already in spot 5 with a good chance of getting in Champions League. Two of their players are back and that is good for the team.
After all I've said, I would not be surprised by the visitors' winning but I'll go with the safer X2 option. The only advantage Marseille have here is tradition but that shouldn't not play an important role here.