The Football Association is not expected to make a decision until later this week.
The date for the replay is likely to hinge on Chelsea's Europa League encounter with Steaua Bucharest at Stamford Bridge on Thursday.
If Rafael Benitez's men lose, the replay is set to be played during the first week of April.
If Chelsea are able to overturn a 1-0 first-leg deficit, then a fixture pile-up would cause a real headache.
The international break rules out United heading to Chelsea for the right to play Manchester City in the last four any earlier, and it is viewed as impractical to cancel Premier League fixtures due to be played this weekend.
It would leave the FA either asking for matches on Easter weekend - United visit Sunderland and Chelsea head to Southampton, both for TV fixtures - to be postponed, or those on the following weekend, which would include the Manchester derby.