The prospect of losing Mendy for the rest of the season severely weakens Pep Guardiola's strategy for the campaign as the left-back was a key part of his summer squad rebuild, with City paying £52m to Monaco for him.
In a brief statement on the official site, City revealed that Mendy travelled to Barcelona to see a specialist doctor, who today confirmed the extent of his injury.
CITY have confirmed Benjamin Mendy will undergo surgery after rupturing knee ligaments - meaning the France ace faces NINE MONTHS out.
"Benjamin will undergo surgery [Friday] in Barcelona, and everyone at the club wishes him a speedy recovery".
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The City boss was forced to play midfielder Fabian Delph at left-back in Tuesday's Champions League victory over Shakhtar Donetsk, with one eye already towards now bringing in a capable deputy for Mendy in the January window.
Asked if City may buy a replacement, Guardiola added: "We will see".
Despite the diagnosis, Mendy was able to keep his sense of humour on Twitter, saying he is "joining Injury FC on loan for a couple months".
He followed it up with: 'Thank you all for the warm messages since the game.