Ferguson believes if they topple the Spaniards, they can go all the way to Euro glory.
He said: "This is the acid test, no doubt about that, and we have to get through to win this competition.
"This is so important to both clubs, we cannot think of failing."
Asked if his United team can emulate those of 1968, 1999 and 2008 and lift the European Cup, Ferguson added: "Yeah, I think so.
"They have a great spirit about them.
"People say this one is not as good as the teams of the past but sometimes you can have foggy recollections of teams from the past - I do myself.
"But this team doesn't know when it is beaten, which is a good asset, and it has some quality players, too."