Ferguson said: "When I see the opponents we are going to play, I hope thunder and lightning hits the pitch and they lose 25-0.
"I always look for disasters happening to our next opponents and look for grains of hope."
No team in Champions League history has overturned a 2-0 first-leg deficit. United also have an unbeaten home record in the competition stretching back three years.
But Fergie said: "We never take anything for granted and we look upon this game as our most important of the season so far.
"We will wait until after the game and see where we are. Then we can maybe start to dream. We have one foot in the semi-final but to get two feet in there we need to approach the match in the proper fashion and win it.
"In Rome, we worked our socks off tactically and with good discipline. That is what we will need again.
"We will not be looking to do anything silly. There are 90 minutes on that clock but it will be ticking against Roma."