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.
The 23-year-old French global, signed for £50 million ($67 million) from Monaco in the summer, will undergo surgery in Barcelona on Friday on the injury he suffered in last weekend's victory over Crystal Palace.
French defender Benjamin Mende was injured by a cruciate ligament in the right knee after the player underwent tests.
Tests have now confirmed he has ruptured anterior cruciate ligaments in his right knee.More news: U.S. slaps sanctions on 8 North Korean banks
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Despite the diagnosis, Mendy was able to keep his sense of humour on Twitter, saying he is "joining Injury FC on loan for a couple monthes".
And he will now undergo surgery in Barcelona on Friday.
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.