Clark severely bruised his foot in the 1-0 win at Airdrie on January 2, but it has not healed, 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 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."
Dean Shiels, who returned from a long stretch out 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."
McCoist has welcomed Lewis Macleod and David Templeton back to training, with both set to face Forfar.
The pair worked with the first team this morning at Murray Park after missing games with calf and thigh strains respectively.
They will join the group at Auchenhowie tomorrow and at the weekend as they prepare to meet the Loons.
McCoist said: "They'll definitely come into the reckoning. It's great to see both boys back and Temps, more than Lewis, has struggled this season with injuries."