"It's been frustrating for them but we need every able-bodied player that we have at the football club and we need players to perform at a high level if we are going to keep our position and maintain it," O'Neill said in the Birmingham Mail.
"At this minute, with Gareth Barry and Stiliyan Petrov having done so well last season and us reverting to a 4-4-2 from when we had three players in the middle of the field, it has been tough.
"Now that John Carew is fit and we have added Emile Heskey to the squad we have some options.
"That gives us food for thought for next year again.
"If we qualify for the group stages of the Europa League we will need bodies, absolutely."