The reigning Scottish champions lost their key forward for the remainder of the season at the end of October and have dropped points in their past two league outings with a scoreless draw against St Johnstone and a 1-0 loss at Kilmarnock yesterday.
McCoist says 'Gers cannot use Naismith's injury as a justification for failing to score in two matches even though he acknowledges how big a loss he is.
"Any team in the country would miss Naismith and we are no different," said McCoist.
"But there is no use sitting here saying we are missing Naismith without doing something about it. We have to do something about it.
"We have to win games other ways. Naismith ain't coming back so we just need to get on with it.
"Naismith being injured is a massive loss but we have other players in the squad who should be able to fill the gap. We can't use it as an excuse because it's not an excuse."