Villa face City on Saturday and will come up against Fulop, the third-choice keeper at Sunderland, following Shay Given's injury.
"I can't do anything about it," said Villa manager O'Neill said. "Do I understand it? No. The Premier League have made the decision and Manchester City have now acquired him.
"It would be hard to imagine Fulop as being a third-choice goalkeeper because they have a goalkeeper who is an international."