The struggling visitors took the lead through Sean Morrison's first Premier League goal but Jordi Gomez struck twice in the second half to send the Latics ahead.
The Royals came again and thought they had seized a hard-earned point from the trip when Ali Al-Habsi put through his own net - but Gomez dramatically completed his hat-trick from 10 yards in the final minute.
McDermott said afterwards: "I thought we deserved a point, and I thought it was the least we were going to get.
"I think if the referee had given the penalty on Jay Tabb we would have gone on to win the game. Someone of Howard Webb's experience and quality, you would have expected him to get that one right.
"When you're 2-2 in the 92nd minute and we've got the ball, you've got to make sure you see the game out. If you have to take a point in this league then take it and move on.
"We have to learn quickly from that. We've seen out the last couple of games really well but unfortunately we haven't seen this one out."