It was an unhappy return for McDermott on his first visit to the Madejski Stadium six months after the Berkshire club sacked him.
"That was tough to take," McDermott said. "I'm not sure why the referee was still playing. I've no idea.
"It was an awful feeling conceding so late but we did it to Brighton earlier in the season. But it won't define our season. We didn't deserve to lose that game but that's what Alfie does and he does it well.
"I'm gutted for the fans who travelled all this way. I've lost in play-off finals before [with Reading] and that was horrendous. It wasn't horrendous but it was not nice and we move on."
McDermott - sacked by Reading in March after more than 12 years at the club, mostly as chief scout - received a warm welcome back from the Reading fans.
"I appreciated that a lot," he said.