Adam Federici, who kept 20 clean sheets in the npower Championship last term, had been struggling with a knee injury, but was passed fit during training.
Royals boss Brian McDermott decided to leave him out of the squad, however, handing McCarthy his first Barclays Premier League start.
"I went training on Saturday and the gaffer pulled me aside and said I'd be playing," McCarthy said.
"He told me it was a tactical (decision). He said he didn't want me to feel that he was just pulling (Federici) out because he was injured, because obviously that would have an influence on the rest of the boys in the team. I was over the moon to make my Premier League debut."