Mendy was forced off in the 29th minute of City's 5-0 thumping of Crystal Palace on Saturday.
The full-back will have surgery with a specialist in Barcelona on Friday before starting what is likely a long and arduous road to recovery.
Bad news guys ll be joining Injury FC on loan for a couple monthes with ruptured ACL... but will be back soon & stronger hopefully
— Benjamin Mendy (@benmendy23) September 28, 2017
Thank you all for the warm messages since the game really appreciate. We keep in touch all behind my #SharkTeam now #cmoncity
— Benjamin Mendy (@benmendy23) September 28, 2017