The Baggies were in cruise control 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. 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. Here we are on the right end and I am happy.
"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."
- Sports & Recreation
- Pavel Pogrebnyak
- Adam Le Fondre
- Jimmy Kebe
- Romelu Lukaku