O’Connor said: “It is a huge game.
“It is a chance for us to win something. A win will breed confidence and take us into the United game on top form. We must make sure we do not take Rotherham lightly in any way.
“Our players have started to believe they can compete in the Premier League but we have a strong squad and will be using a few other players for this one.”
Boss Tony Pulis said: “I really, really want to go into that game focused and try to reach the quarter-finals.
“I’m not even thinking about Manchester United.”