Guardiola hits out at schedule after serious De Bruyne injury
De Bruyne limped off early in City's 3-0 win over Burnley to open the Premier League season on Friday.
The 32-year-old had suffered a similar injury in the Champions League final just over two months ago as City beat Inter Milan to become European champions for the first time.
"It's a serious injury," said Guardiola ahead of the UEFA Super Cup between City and Sevilla in Athens. "We have to decide surgery or not surgery."
He added: "Surgery I think will be three or four months out."
De Bruyne joins a list of high-profile players to go down early in the new campaign.
Chelsea's Christopher Nkunku had to undergo knee surgery on an injury picked up on a poor pitch during a pre-season friendly in Chicago.
Arsenal's new £40 million ($50 million) defender Jurrien Timber and Aston Villa duo Tyrone Mings and Emi Buendia have all reportedly suffered anterior cruciate ligament injuries.
"In my first pre-season with Barcelona we had 25 days of preparation before the first game, which was a Champions League qualifier. This season we had four or five before the first game," added Guardiola.
"Look at all the cruciate ligament injuries there have been in all the big leagues this start of the season. The players don't rest either physically or mentally.
"They make them travel to Asia or America for the club's purposes and play really hard games on bad pitches and the players get injured, or will get injured, because they don't have these 25 days to prepare."
City have already lost two of their most potent attacking players from last season as Ilkay Gundogan joined Barcelona and Riyad Mahrez departed for Saudi club Al-Ahli.
The English champions have been linked with a move for West Ham's Lucas Paqueta, but Guardiola refused to be drawn on whether they will now be forced into the transfer market for an attacking midfielder.
He still has the likes of Phil Foden and Julian Alvarez to call upon to deputise, but said De Bruyne's skill set is "irreplaceable".
"Kevin has specific qualities. To lose it for a long time is really tough for us.
"At the same time we have to look forward. The skills for Kevin is irreplaceable but we have players with different talent."
