The striker was given two yellow cards for diving in the 0-0 first-leg draw with Hamilton at Tannadice last night.
Television replays showed Murray had been caught by Accies defender Scott McMann inside the box in the 77th minute but, instead of giving a penalty, referee Steven McLean, seemingly on the advice of his assistant Graham Chambers, sent the striker off. Murray had already been booked in the first half after going down under a challenge from Massimo Donati.
It’s understood McLean would be prepared to change his decision but can’t do so until next week. However, Murray is free to play as long as United lodge an appeal today.
McKinnon said: “We are disappointed. It was a stonewall penalty. It was disappointing from the referee. We will appeal tomorrow morning.
“He [Murray] is a massive part of the team. He is in form, scoring goals. It will be an injustice if he is not back in the team on Sunday.
“It happened and we will take it on the chin. But it is a big, big decision. We could have gone up 1-0. We won’t dwell on it providing Simon can play on Sunday.”
McKinnon also confirmed midfielder Wato Kuate had played his last match for the club after he was involved in an on-field spat with team-mate Mark Durnan near the start of the second-half. McKinnon immediately substituted him.
“I did not speak to him afterwards,” said McKinnon. “I spoke to the rest of the players with regards to their efforts. They gave everything tonight. I was more focused on the positives than negatives.”
Hamilton manager Martin Canning credited his injury-hit team for holding United. Influential midfielder Darian MacKinnon picked up a booking and is suspended for the second leg.
“Dougie Imrie will need to shake off a rib injury.
“I haven’t seen the penalty back yet,” said Canning.
“I see Scott has gone for the ball and Murray nicks in front but I didn’t see if there was any contact or not. I have heard people say it should have been but we have had a few against us, so if it was and it went in our favour then it’s a nice change.”