Ziyech, who joined Chelsea from Ajax earlier this summer, twisted his knee during the Blues' recent friendly against Brighton last month, which ended in a 1-1 draw.
It were goals from Harry Kane, Heung-Min Son and Dele Alli that ended Chelsea's unbeaten start to the season. The defeat also allowed Tottenham to leapfrog Chelsea into third place.
"I just twisted it a bit. "It's a long season. We want to close the gap with the teams above us so we need to win games, and step by step try to be the best team in England. "I always enjoy my football.
We are going to miss him - maybe for the rest of the season. "It is a worry for us. We hope it is not a big issue. But there is not to much time to recover. He twisted his ankle so we will see how it reacts in a few hours."
Expected return date: Aston Villa (H) - September 24 Mykhailo Mudryk Mudryk seemed to have sustained a calf injury in Ukraine's 1-1 draw with England last Saturday.