Christian Benteke powered home a header with 10 minutes to go to give Aston Villa a 1-0 victory.
It was another late and crucial goal conceded by the Royals who lost 3-2 at Wigan last weekend.
Reading had looked good for a valuable away point until Benteke nodded home a corner from Ashley Westwood.
McDermott said: "I thought we were going to get a point. I knew the first goal was going to be crucial.
"I thought the shape of the team was good but to lose the goal from a set piece was disappointing.
"I thought we had solved the late goal problem.
"But it's not a six-pointer. It's a game of football and we've got a game of football to look forward to against Manchester United at the weekend."