Mandanda cracked a vertebra in his neck while playing for Marseille against Guingamp on Saturday and was immediately stretchered off the field, before being sent to hospital.
Guingamp's Mustapha Yatabare collided with Mandanda in the 19th minute, with the striker's knee connecting with the neck and head of the 29-year-old goalkeeper, who had rushed off his line to smother a dangerous pass.
Club doctor Christophe Baudot said after the match that Mandanda would not discover his World Cup fate until he had seen a spinal specialist, but it was confirmed on Sunday that he would be required to wear a neck brace for three weeks, taking him up to less than a week before the tournament gets under way in Brazil.
"Following the collision with Mustapha Yatabare during the match OM-Guingamp, Steve Mandanda has a stable crack the first cervical vertebra and whiplash," said Baudot.
"This will force him to wear a neck brace for three weeks and to follow an equivalent period of rehabilitation."
France begin their World Cup campaign against Honduras in Porto Alegre on June 15.
