Sergio Aguero only recently recovered from a knee problem but now faces a month on the sidelines after picking up a muscular injury in Saturday's draw at West Ham.
Aguero, who was playing just his third match since June, was withdrawn at half-time and Guardiola is hoping the 32-year-old returns soon.
Guardiola said: "When you are four or five months out (with) knee problems, always you have a risk when you come back of injuries.
"We tried to handle it as well as possible, just 50-55 minutes a game but it was not enough to get fit for the next games. He will be back as soon as possible."
Guardiola said: "Kevin (De Bruyne) is back again and played a few minutes against West Ham. Aymeric is back too, Nathan is so close. Gabriel is close - one week to 10 days to come back. The rest will be two or three weeks.
Asked if he had a clear idea of how long Aguero might be out, Guardiola said: "It depends on the injury. When it is minimum it will be 10-15 days. If it is longer it will be three weeks to a month. I don't know."
Predicted Manchester City XI to face Marseille (4-3-3): Ederson; Walker, Dias, Laporte, Cancelo; Rodri, Gundogan, De Bruyne; Mahrez, Sterling, Foden