The two Premier League giants were drawn together in the last-16 of the competition.
Chelsea reached the fifth round after a 3-0 win over Sheffield Wednesday on Sunday thanks to a brace from Willian.
Manchester United had secured their spot in the next stage of the competition by beating Arsenal 3-1 on Friday night.
Though the journey home from the Emirates would have been a happy one, a shortage of late-night trains made it a tricky trip.
And it seems United will have the same problem getting to and from London on the weekend of February 15-18.
The tie will not be able to be played on the Friday or Saturday with Chelsea in Europa League action on Thursday February 14.
The Blues play Malmo in that competition meaning their clash with United will be on either on Sunday or on Monday evening.
There is already planned engineering works for the late night trains from London Euston to Manchester Piccadilly on the Monday night.
And there are generally fewer services on a Sunday too.
Some fans are already concerned about arrangements for the crunch FA Cup tie.
One tweeted: “Going for a Monday night! BBC no chance for the fans to get home.”
While another added: “Monday would be a f***ing nightmare.”
And a third posted: “Monday can f*** off and for that reason you just know it will be Monday.”
Chelsea beat Manchester United in last season’s FA Cup final.
The Blues won thanks to Eden Hazard’s first-half penalty in Antonio Conte’s final match as Chelsea manager.
The last meeting in the competition at Stamford Bridge saw N’Golo Kante score the winning goal for the hosts in 2017.