Mendy has made five appearances in all competitions for City in 2017-18, providing one assist.
The Manchester City left-back underwent successful surgery in Barcelona on Thursday after sustaining anterior cruciate ligament damage last weekend against Crystal Palace - the same heartbreak which brought a cruel end to Gundogan's first season at the Etihad Stadium.
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"After initial tests in Manchester at the start of the week, Benjamin travelled to Barcelona to see a specialist doctor, where further examinations of the knee confirmed the extent of the injury", read a Manchester City statement.
He wrote, "Bad news guys".
"It is really frustrating for him, first as a person, but like what happened with Ilkay Gundogan previous year, I am sad for him", manager Pep Guardiola said following his team's Champions League victory over Shakhtar Donetsk on Tuesday.
Mendy's injury is a major setback for City as it comes just before their trip to Chelsea on Saturday and the prospect of losing the 23-year old for the rest of the season severely weakens Guardiola's strategy as the left-back was a key part of his summer squad rebuild.