Fletcher handed in a transfer request and did not feature in the weekend's defeat at Leeds, amid rumours he will soon be joining Sunderland in a £15million deal.
And O'Neill maintains the sooner new men can come in, the better.
"We would have liked to have had some players in before the Arsenal game, but that did not happen," he said.
"We have a lot of spirit about us, but lack quality in some positions. We need some better attacking options."