Clark, who saw his side slip out of the play-off places, said: "Young Nathan has shown a lack of discipline and got what he deserved without a shadow of doubt.
"We will discipline him because his lack of discipline has possibly cost the side. He got involved in a bit of banter before that and it was a very reckless challenge.
"This is his first foray into first-team football and everyone makes mistakes, even senior players. He will be disciplined under the club code of conduct, but he will be helped as well.
"The first 26 minutes have cost us the game when it was 11 against 11. After the sending off I thought we were excellent. We have not sat down in the dugout after half-time and we were 2-0 down and it was from our own kick-off as well."