The home leg of Manchester United's Champions League quarter-final tie could be reversed in the event of a clash with that involving Manchester City, depending on the outcome of Friday's draw, Uefa said on Thursday.
The two Manchester clubs are through to the last eight alongside two other Premier League sides, Tottenham Hotspur and Liverpool.
Unlike in previous rounds, there is no longer any country protection in the draw, meaning teams from the same domestic league can now face one another. That throws open the possibility of a Manchester derby in the quarter-finals.
However, if they do not face each other, United will have to reverse their fixture if they are drawn at home in the same week as City.
"Following a decision made by the relevant local authorities, Manchester City and Manchester United cannot play at home on the same night, nor on consecutive nights," said Uefa.
"Should both clubs be drawn within the same sequence (home or away), the team having finished lower in the domestic league the previous season - in this case Manchester United - will be reversed in accordance with the Uefa Club Competitions Committee principles."
The semi-final draw will also take place on Friday, and Uefa added: "The calendar for the semi-finals will be communicated to all parties on April 18, 2019, following the return legs of the quarter-finals."
Juventus, Barcelona, Porto and Ajax complete the Champions League quarter-final line-up.