Saints have not beaten their local rivals at Fratton Park since 1984.
The Austrian said: "Sure you can be a legend.
"It doesn't matter if you are a League One team or a Premier League team. In that moment it's not interesting.
"It's about this one game and you can be a big hero if you decide it."
He added: "I think this is the best answer of how important it is in what position you are in the table going into a cup game. It doesn't matter.
"It's one game you have to win and in one game in football everything is possible. That's why I expect the most difficult game for us.
"If we are prepared for this we have a big chance to win there."