The Ivory Coast international, 25, suffered the injury during a training session last Thursday while on national team duty.
Saint-Etienne coach Christophe Galtier said his club had not been made aware of the former Leicester and Leeds winger’s injury until early this week.
They are also without full-back Jeremy Clement, who suffered a horrific broken leg earlier this month, for which Nice's Valentin Eysseric was banned for the rest of the season.
Les Verts lie fourth in Ligue 1 after an impressive season so far – they also face Rennes in the Coupe de la Ligue final on April 20, and are in the quarter-finals of the Coupe de France.
- Sports & Recreation
- Christophe Galtier