France centre Mathieu Bastareaud was undergoing tests on Monday on a calf injury that threatens his participation in next weekend's Six Nations tie against Italy at the Stade de France.
Bastareaud, who has featured in France's first three matches, injured himself in a gym session at the team's training centre on Sunday.
The Stade Francais star played in Saturday's 0-29 loss at home to Toulouse.
Toulouse scrum-half Frederic Michalak has been ruled out of the Italy game because of a knee injury and will be replaced by Biarritz's Dimitri Yachvili.
France are the only team in the running for the Grand Slam and play Italy on March 14 and England on March 20.
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