After initial tests in Manchester, Mendy travelled to Barcelona to see a specialist, where the extent of the damage was confirmed.
He joked on his Twitter feed that he would be joining "Injury FC" on loan for a couple of months but hoped to return stronger.
This is a blow to City, because it means their first-choice left back won't play until at least the semi final stage of this year's Champions League.More news: Moscow regards situation in Catalonia as Spain's internal affair
But Guardiola insisted yesterday that he is not concerned by his players' off-field activities.
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.
"I don't know, we will wait until Thursday, but I am pretty sure it will be longer than we would like, unfortunately".More news: Free Coffee! Friday is National Coffee Day, here's where to snag deals
And the 23-year-old still had time for some lookalike bants as he was wheeled into surgery after he tore his ACL.
Delph again deputised for Benjamin Mendy, who is out until April with a serious knee injury.
Manchester City will move for highly-rated Napoli left-back Faouzi Ghoulam in the January transfer window, according to reports from his homeland of Algeria.
Midfielder Fabian Delph filled in at left-back during the 2-0 win over Shakhtar Donetsk on Tuesday and Danilo can also play there.More news: Newcastle United vs Liverpool - betting tips and predictions
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