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, even though UEFA prefers major countries not to arrange domestic fixtures at the same time as their competitions.
If Chelsea are able to overturn a 1-0 first-leg deficit, then a fixture pile-up would cause a real headache.
United boss Sir Alex Ferguson said after Sunday's 2-2 draw at Old Trafford: "I don't know when the replay will be played, probably May!"
Chelsea would need to play a Europa League quarter-final in the two midweeks immediately prior to the confirmed FA Cup semi-final dates of April 13 and 14.
The preceding 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.
The FA might be loathe to do that, though, because it merely stacks up a problem for later in the season, with matches from this weekend and those due to be played when the semi-finals take place already needing to be re-arranged.
That would force Chelsea into slotting in the replay at the start of a midweek that also involves a European commitment.
Even that is not straightforward as Fulham are due to entertain QPR on Easter Monday, April 1, and local police are unlikely to sanction a cup tie on the same day.
The following Monday is when United are set to entertain City, although that fixture was set to be brought forward to the previous Saturday had Ferguson's men made it past Real Madrid in the Champions League.
An additional complication is whether Chelsea get drawn to play at home first, or away in the Europa League draw when it is made on Friday.
Nothing can actually be confirmed until Thursday night (if Chelsea go out) or Friday morning (if they don't) - with the exception of moving back more PL games.- FairyNuff