However, Ferguson has insisted that his side must play to the best of their ability to see off Reading on Monday evening.
Ferguson told reporters: "We should not get carried away, particularly in the FA Cup when anything can happen. You don't want to be a shock.
"In many people's eyes, Monday would be, 'yes, it's Reading, we have just been to Madrid, it was a great night, Monday will take care of itself'.
"Monday can't take care of itself. We have to take care of that."
Ferguson has already revealed that he will make changes to his side for the match at Old Trafford.