James Robson of the Manchester Evening News confirmed Mendy was hurt during the clash against the Eagles:
James Robson @JamesRobsonMEN
Mendy has gone off. Couldn't shake off the knee injury picked up after challenge on Townsend2017-9-23 14:29:05
After the match, manager Pep Guardiola said Mendy would have a scan to determine the severity of the injury, and he hoped it wasn't a serious ligament injury, per Robson.
Mendy became the most expensive defender of all time when he completed a £52 million move to the Etihad from AS Monaco this past summer.
Although a defender by trade, Mendy illustrated his attacking value en route to winning the Ligue 1 title with Monaco last season by picking up 11 assists in 39 total appearances, an audacious tally for a full-back.
There aren't many left-backs in Europe, never mind the Premier League, who can boast the same efficacy going forward with the ball. He possesses an attacking might that Guardiola is sure to miss.
It's for this reason Guardiola also signed versatile Danilo from Real Madrid, with the Brazilian at times filling in on the left during his stay at the Estadio Santiago Bernabeu.
Veteran left-backs Gael Clichy and Aleksandar Kolarov both left the Etihad over the summer—for Basaksehir and AS Roma, respectively.
Mendy looks likely to miss City's upcoming encounters against Shakhtar Donetsk and Chelsea, for which Danilo could be considered his replacement. Guardiola could also switch to a back three to lessen the blow.