And the decision was so tough that not even the Red Devils themselves could come to an agreement.
Some fans took to Twitter to slam Michael Oliver for pulling out two bookings for the Spain international in the space of just 15 minutes.
Others chose to single out the midfielder for what were a pair of rather unnecessary fouls - but who is on the money?
That's an idiotic decision from the referee. A fantastic looking FA Cup tie and he's thrown it to the bin. Ander Herrera sees red.— The Man Utd Way (@TheManUtdWay) 13 de marzo de 2017
Anyone really think those two fouls by Ander Herrera justified a red card given the way Michael Oliver had refereed this game? #CHEMUN— Duncan Castles (@DuncanCastles) 13 de marzo de 2017
Conte aggressively cheering Herrera's red card.— Miguel Delaney (@MiguelDelaney) 13 de marzo de 2017
'Mourinho, your thoughts on Ander Herrera's red card?'— Mohamed ElNneny (@ElNnenyM) 13 de marzo de 2017
"Chelsea have an easy schedule because they're not in Europe." pic.twitter.com/XMOF1qloXh
@rioferdy5 Herrera was a red card all day long so shut up— Luc (@LucoCarter) 13 de marzo de 2017
Red card for Herrera. Oliver has just ruined this game, never a second yellow card. #mufc— Aidan Walsh (@AidanMUFC_) 13 de marzo de 2017