Long-time leaders Lille's 1-1 draw at FC Lorient on Sunday, the third league game in a row they have failed to win, means Marseille can take over at the top by a point with six matches left with victory in their game in hand.
"The news is not good. Mbia has a problem with his adductor muscle, he is not set to return until mid-May," Marseille coach Didier Deschamps told reporters on Monday.
The injury to the influential Cameroon midfielder comes when Marseille are battling potentially crippling suspensions.
Striker Loic Remy will again be banned against Nice while defender Taye Taiwo, who scored the only goal in Saturday's League Cup final win over Montpellier, could face a maximum six-game suspension.
After the final, the Nigerian chanted a song including an expletive insulting rivals Paris Saint-Germain but it is unclear when the French league will rule on his case.
"In that atmosphere, I sang with the fans not really knowing what I was saying," Taiwo told reporters on Monday having already apologised to PSG.
Charles Kabore also picked up a booking on Saturday and must serve a suspension, although he looks set to be available against Nice with his ban likely to come into force in the May 7 clash against title rivals Olympique Lyon.
Third-placed Lyon, seven points behind Lille, also have a game in hand, at home to Montpellier on Wednesday.
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