Manchester United or Manchester City will be forced to play their upcoming Europa League matches on Tuesday afternoons.
The North West rivals were dumped out of their respective groups in the Champions League on Wednesday night, resulting in a drop to the Europa League.
While both sides were preparing to face European opponents on Thursday nights, a decision is expected to be made next week declaring which club will have to compete earlier in the week.
UEFA rules state that any team within a 31-mile radius of each other cannot compete on the same day, which means that one side could have their dates moved or have a fixture reserved.
The Premier League rivals will be seeded in next Friday's Europa League 32-club draw.
