Manager Pep Guardiola splashed the cash in the summer with the former Monaco full-back been the final addition to the Catalan's side, but having suffered with injury before his arrival it took until the 2-1 win over Bournemouth before the Frenchman could make his debut for The Cityzens.
After initial tests in Manchester, Mendy travelled to Barcelona to see a specialist, where the extent of the damage was confirmed.
But now, after hearing that he has ruptured his anterior cruciate ligament, he's maintained his playful online persona and made fun of his strife.
"Benjamin will undergo surgery tomorrow (Friday) in Barcelona, and everyone at the Club wishes him a speedy recovery".
He wrote, "Bad news guys".
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.
"It is really frustrating for him, first as a person, but like what happened with Ilkay Gundogan a year ago, I am sad for him", manager Pep Guardiola said following his team's Champions League victory over Shakhtar Donetsk on Tuesday.
The setback leaves City without an out-and-out left-back, and Guardiola has indicated that he could bring in a new signing in the January transfer window.
Jonathan is ESPN FC's Manchester City correspondent.