Ivanovic does not have too much sympathy with United's complaints about Nani's harsh red card during their Champions League defeat by Real Madrid this week.
"But I think that will make them more motivated and we have to be ready for that.
"This is a big test of our character and we need to react after the defeat in Bucharest.
"We have had two days less to prepare for this FA Cup match than United and it is always a very physical game.
"But we have to recover well and be ready for the challenge."