Marta, named Women's Player of the Year a record six times, picked up the injury at Brazil's camp in Portugal and while the CBF did not clarify how serious the injury was coach Vadao told a Globo website she could miss their June 9 opener.
"Our worry is just about the first game," he said, "and whether she has time to recover or not.
"She feels fine. We’ll wait a couple of days for the medical department to give me a more concrete appraisal."
The absence of Marta, who has scored a record 15 women's World Cup goals, would make Brazil's life even more difficult at the June 7 to July 7 tournament in France, where they also meet Australia and Italy in the group phase.
The finalists in 2007 come into the tournament having lost their last nine matches, their worst ever run.
(Reporting by Andrew Downie; Editing by Peter Rutherford)
