Mendy became the world’s most expensive defender in the summer after joining City from Monaco for £52m.
The France international will see a specialist in Barcelona and have surgery on Friday.
“Bad news guys,” Mendy posted on social media. “I will be joining Injury FC on loan for a couple months with ruptured ACL but will be back soon and stronger hopefully.”
On Tuesday, City manager Pep Guardiola said he “did not know” how long Mendy would be out for after his side’s Champions League win over Shakhtar Donetsk.
The Spaniard was asked whether he would consider looking for a replacement left-back in January and replied: “We will see in January.
“We thought next summer about looking for another left-back – another replacement – but we will see what we do.
“We have alternatives in our squad – Danilo, Fabian Delph, Fernandinho, can play there, so we will see as we go along.”
Mendy will travel to Spain to see Ramon Cugat, a close friend of Guardiola, who treated City duo Vincent Kompany and Kevin de Bruyne last season.
BBC Sports