Fellaini came off the bench to score last night at West Bromwich Albion.
But McGrath told talkSPORT: "United fans haven't seen the best of him because he has never really played in his best position. Only probably once or twice has he played up front.
"He showed last night it is possible for him to have a presence at Manchester United but, with the abundance of talent we have got up there, he may never get that chance.
"It is a pity for him that he is put in midfield. He did change the game last night because he put himself about, but I don't think his future would be at Manchester United, I really don't."
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