The Royals boss lamented the fact that Manchester United were able to bring on a player of the quality of van Persie in the 2-1 FA Cup fifth round loss at Old Trafford and revealed he politely asked him to refrain from coming onto the pitch when the score was 0-0.
"It makes you think when they can bring on players like Robin van Persie as well. When he was warming up I did speak to him and told him to sit down," McDermott said.
"We gave them a real run for their money," he added.
"In the last 20 mins we played some very good football.
"But they have some fantastic players and Nani is one of them.
"It is the first time I have managed here (at Old Trafford) and it was good for me personally and my players to come here.
"We have to come back here in the league and will know a bit more about what to expect.
"We really wanted to put on a performance for our fans and we have done that.
"Now we can look forward to Saturday's game against Wigan."