The United captain is being linked with Real Madrid this week.
And McCoist told talkSPORT this morning: "He's away. And not only that, there'll be a knock at the door from Fernandes himself, saying, 'Excuse me, I've got a chance to go to Real Madrid here,' so they are two massive factors, no doubt about it.
"But it goes without saying, he has been there only shining light this season, a lot of people have questioned him over whether he should be captain.
"I'll be quite frank with you, some of his - tantrums is far too strong a word - but some of his reactions to things are sometimes I don't think are sometimes befitting of a captain.
"But I tell you what you cannot argue with, the way he's performed and led by an example as a captain should, so he would be an absolute savage loss to Manchester United."