The winger was dismissed for elbowing Forest midfielder Henri Lansbury in the stomach right in front of the match official.
"I've spoken to him about it. I wouldn't tell you what I've said, but I've spoken to him, of course," McCarthy is quoted as saying by the East Anglian Daily Times.
"There is a disciplinary code of conduct. There are rules in place that mean, if you get sent off, then disciplinary action is taken. He's accepted that, come in and held his hands up."
The red card was the third that Martin has received during his career.