Pompey managed to send United out of the FA Cup last season in the quarter-finals at Old Trafford.
“We showed last season that anything can happen. That is the beauty of football,” he said in the Daily Mirror.
“You cannot go into a match thinking you are going to lose. We need the points and this is another opportunity to take three points.
“It does not matter if you are playing United or Bolton, you have to go out and fight for the win and that is what we will do.”