De Bruyne limped off in the closing minutes of City’s 2-0 victory at the Etihad Stadium after falling awkwardly in a clash with Timothy Fosu-Mensah.
The 27-year-old had scans on Friday that revealed the left knee injury is serious enough that he is unlikely to be back in action until mid-December, although he does not require surgery.
De Bruyne only recently returned from another knee problem that forced him to miss two months this season, limiting the Belgium international to five appearances.
His absence during a busy period is a major blow for Premier League leaders City, who face crucial Champions League fixtures, the Manchester derby and a trip to Chelsea before De Bruyne is likely to be fit.
“Manchester City midfielder Kevin De Bruyne suffered knee ligament damage in his left knee during Thursday night’s Carabao Cup fourth-round win over Fulham,” a statement on City’s website said.
“The Belgian underwent scans on Friday at the CFA. No surgery is needed but he is expected to be out for between 5-6 week
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