Confirming the news of Mendy's injury, the club said that has ruptured the anterior cruciate ligament in his right knee and that he would now undergo knee surgery.
Mendy underwent initial tests in Manchester earlier in the week before travelling to Barcelona for further testing, City confirmed the injury on Thursday but it is not known how long the Frenchman will be out for as he is expected to undergo surgery in Catalonia on Friday.
The France global will undergo surgery on Friday and, although City have not said how long he will be out, players can take up to nine months to recover from such an injury.
City didn't give a likely timeframe for Mendy's absence.
He wrote, "Bad news guys". 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.
The Spaniard was asked whether he would consider looking for a replacement left-back in January and replied: "We will see in January".