The former Chelsea man was selected to start just his fourth Premier League game of the season at the Etihad Stadium, benefitting from the shock snubs of Alejandro Garnacho and Marcus Rashford.
Mount joined Bruno Fernandes, Amad Diallo and Rasmus Hojlund as Man United's offensive contingent, having quickly been identified by Ruben Amorim as a player who could flourish in his system.
However, the 25-year-old's perpetual fitness woes continued at the home of the English champions, as with under 15 minutes on the clock, he sat down on the turf having picked up an as yet unspecified injury.
After being attended to by a member of the medical team, a visibly miffed Mount trudged off the field to be replaced by Kobbie Mainoo, and he was given condolences by both Man United and Man City players as he made his way off.
Mount headed straight down the tunnel for assessment on his injury, the severity of which remains unclear at the moment, but Amorim will be expected to deliver an update when he conducts his post-match media duties.
Mason Mount's injury record before and after joining Man United
Even the biggest Man United-hating Man City supporter must have felt a shred of sympathy for Mount as he bowed his head on the turf, no doubt thinking 'not again' as he prepares for another possible spell in the treatment room.
Sunday's derby was just Mount's 32nd appearance in a Man United shirt since joining in the summer of 2023, and the Englishman has already missed 37 games through injury over the past year and a half.
Mount firstly sat out six matches in the earliest weeks of his debut season, before a serious calf injury rendered him unavailable for almost exactly four months between November 2023 and March 2024.
Hoping to put his fitness woes behind him for 2024-25, Mount instead picked up a new hamstring concern in August, missing another three matches, before a combination of a head injury and a knock saw him sit out seven games between October and November.
A statistic that makes for grim reading for Man United fans, Mount has already missed more games for the Red Devils than he did in his entire Chelsea career, only sitting out a total of 18 matches for the Blues and never more than seven at a time.
Whether different training practices, a failing body or sheer bad luck are to blame for Mount's worsening injury record, the Englishman has every right to be inconsolable, and there are fears that he could now be set for another stint on the sidelines during a hectic festive period.
Written by
Ben Knapton