The midfielder was sent off for a clash with Jonas Olsson.
Petrov said: "It was nothing.
If you look at the sending-off, Olsson punched Herd on the shoulder. It's a man's game and you expect to have physical battles.
"Chris was shocked. He's a young boy and didn't realise he was going to be sent-off.
"He's been doing well for us but that's how he is going to learn. He's a strong character and we support him. We take the blame as a team."