The left-back suffered the injury in Saturday's win over Crystal Palace, but the severity wasn't clear until now.
Per the report, Mendy had initial tests done on his knee in Manchester and flew to Barcelona to see a specialist.
The Frenchman didn't lose his sense of humour: He announced on Twitter he'll be joining "Injury FC," advised a fan to sell his FIFA 18 One to Watch Card and proclaimed his intention to become a professional FIFA 18 gamer.
As City's top option at the left-back position―and the only real specialist―Mendy was a highly anticipated arrival this summer. He enjoyed a fine start to the season, and City worked well with him in the starting XI, so manager Pep Guardiola will have some real tinkering to do.
Per Goal's Shane Burns, he's unlikely to play again for City this season and could miss the 2018 FIFA World Cup as well.