The French worldwide suffered the injury in Saturday's 5-0 Premier League victory over Crystal Palace at the Etihad Stadium.
Steve Nicol assesses how Man City should proceed if Benjamin Mendy misses an extended period of time with a knee injury.
City said on their website (www.mancity.com) that the player had travelled to Barcelona to see a specialist, after initial tests in Manchester, with the results on Thursday confirming the extent of the injury.
Tests have now confirmed he has ruptured anterior cruciate ligaments in his right knee.
Mendy will undergo surgery in Barcelona on Friday.
But Mendy offered hope that he could be back much sooner when he wrote on Twitter: "Bad news guys".
The offseason arrival of Mendy and right back Kyle Walker, two of the most attacking full backs in Europe, has added a new dimension to City's play going forward.
The City boss also did not rule out buying a new left-back in the January transfer window after being linked with Southampton's Ryan Bertrand over the summer. City's midweek match against Shakhtar Donetsk saw Fabian Delph fill in Mendy's position, and the only other suitable option ahead of the club's weekend encounter with Chelsea comes in the form of natural right back Danilo.