Johnson believes United's current difficulties could open the door for the Black Cats to snatch a foothold in the tie.
He said: "Hopefully we can try to catch them when it is probably the most difficult time they have had in 15 years.
"If there's any time to play them, it's probably now, so we can try to make the most of that.
"But they are still a top quality team with top quality players and the key is to go to Old Trafford not down.
"No matter who you are, you have got to try to win the cup games and try to progress and win silverware.
"The league is the main aim for us, but as long as we are in the cup, we will enjoy it."