The striker was dismissed eight minutes before half-time for kicking Greg Halford after the ball had gone out of play.
Goals from Radoslaw Majewski and Henri Lansbury then secured maximum points for Forest to leave Powell frustrated.
"I have seen it since and it is definitely a red card. It was the red mist for Yann. He does get a lot of knocks and is not always protected but no excuse," he said.
"We lose him for three games now and he was immense for us in midweek.
"I was a bit perplexed at the time as to why the referee consulted the linesman on the far side, which seemed a bit bizarre.
"The sending-off made it even harder for us as it was difficult enough when it was 11 v 11 as Forest settled very well and used the ball well.
"We really needed to get to half-time and get reorganised but when you are down to 10 it makes it harder."
The Addicks have only won four home games this season and have failed to record an effort on goal for 40 minutes.
Considering that form, Powell jokingly said: "I want to play all 12 games away now.
"I know that would not be fair on our fans but we have to try to work out what it is. We can't put our finger on it but we have to sort it out - we are trying to turn it around."