David Templeton looks set to spend a number of weeks on the sidelines with ankle ligament damage.
The winger was forced out of Saturday's goalless draw against Annan in the Third Division and has since had an X-ray.
There is no break and further scans will be required to put an exact timescale on the 23-year-old's return, but McCoist admits the signs do not look promising.
Templeton was only recently recruited from Hearts and news of his injury has come as a major blow for Rangers.
"David has obviously got a very sore one. The good news is that it's not broken," said McCoist.
"He has been for an X-ray but there is massive swelling and there looks to be a good bit of damage to the ligaments unfortunately.
"We are waiting on the swelling going down and then David will get a scan in the next few days.
"I couldn't put a timescale on the return from injury unfortunately but it's not great."
McCoist added: "We were delighted to get wee Temps in because he was without doubt our number one target in the window. He creates things and scores goals.
"It's a bitter blow for us but it's something we have to handle and get on with."