City said on their website (www.mancity.com) that the player had travelled to Barcelona to see a specialist, after initial tests in Manchester, with the results on Thursday confirming the extent of the injury.
Before any deal could potentially be done, Ghoulam is due to face City twice in the Champions League, with Napoli also in Group F. Like with Mendy last season, this will give Guardiola an opportunity to watch the player up close before potentially making a cut-price January bid for him.
Given City's trip to Chelsea, questions may be asked why Aguero was in the Netherlands 48 hours before such a serious match.
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City have an overwhelming amount of firepower going forward, but Mendy's injury leaves them weaker at the back and vulnerable on the left flank, which could jeopardise their title chances.
"Bad news guys - I'll be joining Injury FC on loan for a couple months with ruptured ACL. but will be back soon & stronger hopefully", the 23-year-old tweeted.
City had signed the left-back for a world-record fee (for a defender) for £52 million in the summer from Monaco and the powerful Frenchman had settled well in Manchester and made the left-back slot his own with a series of impressive performances.
According to Le Buteur, City have set their sights on Napoli left-back Faouzi Ghoulam.