Reading seemed on course for their first Barclays Premier League win of the season after goals from Jimmy Kebe and Noel Hunt in the space of four second-half minutes.
Ba had equalised in between the two Reading strikes with a magnificent volley and then stole a point for Newcastle in controversial circumstances six minutes from time.
The Senegal striker went to meet Shane Ferguson's cross with his head but missed it and the ball deflected off his hand past the helpless Reading goalkeeper Alex McCarthy.
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."
Even when Newcastle had drawn level, Reading had the chances to win but Jason Roberts was denied by an excellent save from Steve Harper and Jobi McAnuff hit the post.
"When it got back to 2-2 we hit the post and Robbo had a real good shot which the keeper has made a really good save from," McDermott said.
"We played on the front foot, with tempo and identity. With 33 games to play that is how we have to perform in this league.
"It was much better. We felt at West Brom we were hoping to get a result. Today we took the initiative and we thought we were going to win."
Pardew worked with McDermott in his time at Reading and was honest in his appraisal of Newcastle's performance.
"I have seen it again and it definitely hits Demba's arm," Pardew said.
"I feel for Brian, he hasn't got the win that perhaps he deserved today.
"I am not sure I will say sorry because professionalism won't let me do that but I will say his team deserved to win, I have no problem with that.
"The performance level from us was down. We can't keep relying on our spirit and a bit of quality to get us these points.
"We have got to find a way of getting rhythm in our play but Reading did a good job of upsetting that today. They didn't give us a moment's piece. They were all over us.
"Some good teams will get beaten here and we were fortunate we weren't one of them."