France international Yoann Gourcuff will miss up to three months with a knee injury.
Despite the problem not being as bad as first feared, scans have since shown that the 26-year-old has suffered damage to his ligaments.
"At the time it happened, I feared he had torn his cruciate ligaments. His ankle didn't move along and his leg just seemed to lock itself," Lyon's club doctor Robert Duverne told Le Progres.
"It is not as bad as expected, as he has sprained his right knee and damaged his lateral collateral ligaments, but it's a serious injury. He could be out for anything between six to 12 weeks."
Yoann Gourcuff has recently been linked with a transfer to Arsenal.