Marmoush was forced off on nine minutes of Egypt's draw with Burkina Faso in Ouagadougou on Tuesday.
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The Egyptian FA have reported the striker suffered knee ligament damage, with X-rays due on Wednesday when the squad returns to Cairo.
Marmoush will be further assessed when he returns to England by City medics ahead of the weekend derby against Manchester United.
Marmoush, 26, has started City's last two top-flight matches - defeats to Tottenham and Brighton - after coming on as a substitute in the 4-0 opening-weekend win at Wolves.
