If he was injured then it is obviously a concern, and not much can be done about it.
If he's missing games because he can't get in the side, I think (given our lack of options in the striking dept) we should still look to play him. Celtic are a stronger side than Scotland, hence he might not be figuring as much as we'd like.
Our (excuse the oxymoron) "strength in the depth" is so weak, that if some our best players aren't playing week to week for their club sides, I don't think we can afford to let it bother us.