The winner arrived in the 38th minute from Nathaniel Knight-Percival, who nodded a cross into his own net.
"Open's a good way to describe that game.....kamikaze if you like," McDermott said.
"It's fair to say that without the ball we didn't do as well as we normally do. With the ball at times we were very good.
"You have to give Peterborough credit because they had good players who pass the ball. They caused us problems.
"We know that as a team we need to be better with the ball, but we're pleased to be in the next round.
"We wanted to get into the next round and we've managed that, which is important."