The Baggies were cruising after Romelu Lukaku had put them 2-0 up but Reading scored three goals in the last nine minutes to snatch victory from the jaws of defeat. Jimmy Kebe stole in at the back post to beat Ben Foster with a header before Adam Le Fondre scored a penalty and Pavel Pogrebnyak bagged the winner in dramatic late turnaround.
"It was a struggle today. We had to stay in the game and they didn't get the third goal, which would have finished the game off," McDermott said. "If you get a goal back, anything can happen. We have been 4-0 up and lost 7-5 this season. We have been on the wrong end of some high scorelines. Today we are on the right end and I am happy."
He added: "The real Reading is the last eight minutes because that is what I have seen over the last three years - a phenomenal character, desire and resilience.
"You don't normally win games when you are 2-0 down. We have a dressing room that is all going in the same direction. It is a big win for us. Pavel's shots-to-goals ratio is phenomenal. He didn't stop running. He is an absolute team man, as they all are, and it was a terrific finish for the third goal."
For 80 minutes Reading seemed destined to be bottom of the Barclays Premier League after QPR's draw with Tottenham. But the unexpected victory has given impetus to their bid for top flight survival.
"We have won two of the last three games in this league and three of the last four (in all competitions)," McDermott said. "We have to keep doing our stuff. Each staff member and player is doing their best. That is all one can ask."
West Brom head coach Steve Clarke looked shellshocked after the game and he was at a loss for words to describe how the Baggies threw away such a dominant position.
"We had good control and I certainly couldn't see the finish to the game that happened," said Clark, who denied it was down to complacency or a sense that his players felt the game was won.
"I am disappointed. I am not sure (what happened). I can't be more honest than that. We should have scored four or five but then we lost a goal and for some reason we lost our shape and we lost our composure and conceded the two late goals that cost us the game."