"Cruciate ligament rupture, six months out," Ait Fana, who suffered the injury in training on Thursday, said on Twitter.
Ligue 1 champions Montpellier have got off to a sluggish start to the season and lie 15th in the standings after eight games.
Get the best features, fun and footballing frolics straight to your inbox every week.
Thank you for signing up to Four Four Two. You will receive a verification email shortly.
There was a problem. Please refresh the page and try again.