The veteran shot- stopper will replace Cammy Bell between the sticks at Ibrox after the Light Blues No.1 was injured in training.
Simonsen has played just one competitive game - a League One win over Forfar earlier this year - since becoming the third keeper on the Gers' books in the summer.
But McCoist is confident the former Stoke and Everton keeper can step up to the plate as the champions bid for a final berth this afternoon.
He said: "Simonsen is in. Cammy got a sore one on the jaw on Tuesday. The decision has actually been taken out of our hands because it is a medical one.
"The good thing is Cammy feels 100% fit within himself. He is desperate to play and is falling out with all the medical staff.
"But, sadly, he can't play because of the complexities of the medical situation.
"Cammy wants to get on with it, but I can totally understand the medical reasons.
"You only need to be wrong once - and that would be far too much.
"As disappointed as I am, I would never say anything against the medical situation.
"Simmo is an experienced goalkeeper. I have said all along that we need two experienced goalkeepers. We felt we had to bring in that back-up.
"Simmo arguably made the best save of the season when he came in against Forfar so he will be fine.
"He has been fantastic here. He trains every day with Jim Stewart and Cammy so we take a great degree of comfort from him being able to step in."
Rangers will also be without Lewis Macleod, Andy Little and David Templeton for the United clash while it will take 'a minor miracle' for defender Lee Wallace to be judged fit to play.
McCoist said: 'It's not ideal because we probably have five out who would play but we are where we are and we need to get on with it."