The 26-year-old Frenchman suffered the injury during the victory at Coventry last week and subsequently missed the home win over Watford on Saturday.
"It is a massive blow with Guy Moussi because it has been confirmed that we are probably looking at 12 weeks," said Forest boss Billy Davies.
"It is a disaster for us. It is bad news for us, for him and the club. We have lost one good player in Moussi."