On Friday, it was confirmed that the 28-year-old had left the Senegal national team camp with a thigh injury.
The news leaves Mendy as a huge doubt for the Blues' game at home to Southampton on October 17.
Kepa Arrizabalaga is now set for another chance as his replacement.
Despite being the world's most expensive goalkeeper, the 26-year-old has so far struggled to meet expectations at Stamford Bridge.
But he will not be short of match practice if called upon for the Southampton game, having kept a clean sheet against Portugal while on Spain duty in the week.
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