Clark suffered severe bruising to the foot in the 1-0 win at Airdrie on January 2 but the problem has yet to heal and McCoist fears the 22-year-old may have a more serious problem.
He told the official Rangers website: "I have to say it's not great news on Nicky at the moment and he's still feeling the foot to be very, very tender.
"He has been for an x-ray and that has showed there's a good bit of bruising but we're now sending him for a scan to see how serious it is.
"We're sitting with our fingers crossed and hopefully it's not too serious but he's wearing a protective boot as a precaution more than anything at the moment."
Dean Shiels, who returned from a prolonged stretch on the sidelines to fire both goals in the 2-0 win over East Fife last week, could also miss the Station Park clash.
McCoist said: "Dean came back in at the weekend and did very well but he has taken a whack in training today. He limped off and headed into the treatment room so I'm not sure how he'll be. We'll have to assess him tomorrow morning."