Manchester City boss Pep Guardiola fears Sergio Aguero has suffered a "bad" injury, which could keep him out for some time.
The Argentine was forced off in the 77th minute of Manchester City's 2-1 win over Chelsea at the Etihad on Saturday and replaced by Brazilian forward Gabriel Jesus.
"Sergio will be bad, I think. Tomorrow we will know it. The feeling is it's muscular," said Guardiola.
Aguero wasn't the only City star to make way because of injury.
Both David Silva, playing in his final season for the Citizens, and summer signing Rodri were replaced by Phil Foden and Ilkay Gundogan respectively.
But Guardiola isn't concerned about his two midfielders, explaining they were taken off because they're not match-fit, and not because they picked up serious injuries.
Guardiola added: "Rodri and David had problems with cramp and being tired; Rodri one and a half months without playing games, David needs rhythm and these games are so demanding."
Manchester City went behind against Frank Lampard's Blues when N'Golo Kante slotted past Ederson after getting the better of Benjamin Mendy.
But Kevin de Bruyne leveled the scores with a deflected effort before Riyad Mahrez produced a moment of magic to beat two Chelsea defenders and fire City ahead.
