O'Neill saw his team outclassed at Manchester United yesterday.
"We would have to be seriously looking to try to strengthen our side," said the Irishman.
"You only have to have look at the back of the programme to see that we don't have great numerical strength.
"But you are going to get injuries and suspensions. It is up to us to cope with them.
"Today was going to be hard enough without giving a couple of poor goals away. But there is still a decent spirit about us.
"We lack many things in the game but the spirit is still there and has not been broken."