The ex-St Johnstone striker suffered a broken cheekbone in the League Cup win over Motherwell in midweek and has been confined to hospital as he recovers.
Sandaza is almost certain to miss the rest of 2012 with the injury, which caused him to leave the field with severe bleeding to his face after an accidental clash of heads with opponent Shaun Hutchinson.
McCoist said: “I’m just gutted for the lad. It’s a blow for Fran himself and I’ve been up to see him a couple of times in hospital.
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“The most important thing is Fran’s health and that he gets himself back to a position where he can play and train.
“Having seen him and spoken to the doctors and surgeons, that is certainly going to be a longer-term thing. He has got a really sore one.”
Unfortunately for McCoist, Sandaza is not the only injury problem for his side going into the Scottish Cup tie away to Highland League side Forres Mechanics on Saturday.
David Templeton and Andy Little are definitely out, with Ian Black and Dean Shiels heavy doubts who are likely to remain on the sidelines too.
McCoist added: "This is one of our worst scenarios, to have five players who are first-team starters struggling.
“We knew this sort of thing would be a possibility and with the transfer embargo coming up, it was important to try to strengthen and get bodies in.
“Unfortunately we never managed to do that but we’re going to have to get on with it. The flip side is that the younger boys who have been coming in have been wonderful.”
McCoist says he expects similar challenges from the non-league outfit to what his side have faced away from home in Division Three, where they are yet to win in three matches.
"Nothing should shock us now as players, managers and coaches,” he said.
“The grounds might change, but the general area of the pitches don't and that's where we have to perform.
"We can't have any excuses. I think we'll get the same type of games from Forres as we've had when we've gone away in Division Three this season.
"There has been no shortage of talent or spirit from the opposition and I don't think we'll get anything different this weekend."