McDermott revealed that Newcastle manager Alan Pardew acknowledged after the game it had been a handball and that Reading deserved the win.
"It is disappointing to say the least," McDermott said.
"I think every angle in the world shows it's a handball. There is no doubt. We knew at the time on the bench because the ball was going across the goal and he has put his arm out and it has gone in the net.
"Whether it was deliberate or not, it's a nailed-on handball and in this league you need to get the big decisions right. It was pretty blatant that one. But he (the referee) hasn't seen it, what can I say?
"You are not allowed to go in to see the referee until half past five. I will go in then and ask him (about the decision).
"He will probably say 'sorry I missed it' and I am sure he will be when he sees it later. The fourth official said something about not too many players appealing. The goalkeeper appealed straight away but that is not what we are about, we don't want players jumping up and down.
"(Pardew) said they were fortunate to get the draw. It was handball. It makes you feel a lot worse. If they score a great goal you can live with that."