"The Palace game will be very difficult for us," said O'Neill. "It would have been fantastic to have been drawn at home, but that would have been asking a lot because we've already had two home ties. It's a tough task for us but we'll go down there and try to get through.
"Crystal Palace are fighting for their lives. It's a great opportunity for them, a great opportunity to keep the club going and they'll be looking forward to it. It's a game that's very much in balance."
Gabriel Agbonlahor and Stiliyan Petrov could both be missing for Villa. Agbonlahor has missed training with a leg problem, while Petrov injured an ankle under a challenge from Nani in the league game against United on Wednesday.