R's winger Wayne Routledge scored the only goal of the game after 82 minutes, with the Royals unable to capitalise after striker Hogan Ephraim's dismissal in the first half.
McDermott said afterwards: "We were in the ascendancy when it was 11 v 11 so it didn't do us any favours when (Ephraim) went off.
"They had one real chance and took it and that's why they will probably win the league this year.
"There were two honest teams having a go at each other.
"It's not a good feeling but there are plenty of points to play for and we are in the running still."
Source: PA
Source: PA