The forward had to be carried off the pitch in that game and speaking after the Carabao Cup defeat to Norwich on Tuesday night manager Neil Warnock confirmed that Mendez-Laing was facing a significant spell out.
"He has done his posterior cruciate ligament, which is good news, my physio says, because if he had done his anterior that’s all season," he revealed.
"Whereas [with a posterior cruciate ligament injury] he will probably hopefully be back in December. It will be about three months."
The 26-year-old was a key part of the 'Bluebirds' side that secured a surprise promotion last season, scoring six goals in 38 Championship games after joining from Rochdale in the summer of 2017.
The loss of such an important player will come as a major blow to Warnock, with Cardiff having failed to score a single league goal in their three games to date.