Royals supporters have begun to question McDermott's methods following a frustrating start to life back in the Barclays Premier League. But McDermott hopes the glamour game with United proves a timely reminder of just how quickly he has transformed the npower Championship winners' fortunes since taking charge three years ago.
He said: "When we were second from bottom in the Championship, if you said to each and every single one of our supporters, 'You're going to be on nine points after 13 games in the Premier League and you're going to be playing Manchester United at 5.30pm on the telly on a Saturday rather than get relegated - would you take that?', they probably would."
He added: "Let's have a sense of realism here. This is where we're at as a club.
"What we've done in the last three years is pretty incredible.
"We've punched above our weight financially.
"But we've got to keep doing that and find another way of finding results and winning and we've done that over the last few years and we need to do that in this league."
McDermott suggested it was the younger fans, who had known little but relative success, who needed a history lesson on a club who have spent nearly their entire existence in the lower leagues.
"You might have a younger generation of supporter who has seen nothing but relative success in the last 10 years," he said. "Then you have an older generation of supporter who knows historically where the club has come from.
"Fans are entitled to their own opinion on the team and that's what makes football what it is."