The defender was ruled out for the rest of the season after rupturing his anterior cruciate ligament in his right knee.
The 23-year-old started his career at the Etihad Stadium strongly after making the £52million switch from French side Monaco.
The Blues conceded only one goal in the five games that Mendy has featured, but his season has now been cut short.
City posted the photo on Twitter with a flexing-arm and blue-heart emoji, indicating their support to the France international.
However, supporters have blasted the club for a gesture that is usually used when somebody associated with a club passes away.
One user @ThePepEra wrote: “He isn’t dead lads.”
While @8mdoowled was also red-faced at the club’s gesture. “embarrassing club,” he said.
@DeadlyDavide echoed those thoughts: “Behave the man is INJURED not DEAD.
And @TheScottKeene added: “Oh Jesus. Come on lads #brokenfootball.”
It is not the first time that City have been ridiculed for such a gesture to one of their injured stars.
Last season the Blues wore Ilkay Gundogan shirts back-to-front during a warm up after the midfielder also ruptured his ACL.
The shirts received a similar response, but City failed to learn their lesson and have found themselves subject to criticism once again.