Since my last post, I've checked the UEFA.com site and the Europa League quarter-finals are set for the 2nd and 3rd April.
If Chelsea do beat Steaua this Thursday, then that rules out 3rd April as a FA Cup replay date as they'll be involved in EL quarter-final action that night (or else the night before).
They'll have to move a PL fixture to midweek to accommodate the replay, which would most likely be played on Saturday 6th April.
The Utd v City match could be moved to Wed 10th Apr as neither Manchester team is in European competition that week.
Chelsea's scheduled match for Sat 6th April (home v Sunderland) would then be moved to Wed 17th April, a date still free, I think.
All of this is of course dependent on the outcome of Thursday's match!