Marmoush was due to have an X-ray on his arrival in Cairo on Wednesday after he was substituted in the fourth minute of Egypt's goalless draw with Burkina Faso in a World Cup qualifier in Ouagadougou on Tuesday.
He sustained the injury following a tackle and tried to play on but eventually had to limp off.
The Egyptian Football Association said in a statement that the national team doctor had "confirmed that Omar Marmoush's injury is a bruised knee ligament, and the player will undergo an X-ray upon his arrival in Cairo on Wednesday morning."
City host Manchester United on Sunday with both sides needing a win after unconvincing starts to the new Premier League season.
Marmoush, 26, has scored eight goals in 28 games for City in all competitions since joining from Eintracht Frankfurt in a £59 million transfer in January.