Rodri has travelled to Barcelona, Spain, to see a specialist following initial tests in Manchester with no timescale being put on the player's recovery.
Following City’s victory over Watford in the Carabao Cup this Tuesday Guardiola spoke on the Spaniard and when his potential return could be.
"Still we don't have the definitive (diagnosis) but he will be out for a long time - a while," said the City boss. "But there are some opinions that maybe it will be less than we expected.
"We are waiting for the last phone calls from him and the doctors for what definitely he has and the type of surgery he has to get.
"We expect that tonight, tomorrow we will know exactly."
City were beaten in four out of the five games Rodri missed last season, while he has lost just once in 50 appearances in all competitions.
His injury came after he spoke about the busy schedule top players are having to face, talks of player protests have started and now with Rodri out they could begin sooner than expected.