The French striker was shown a straight red card eight minutes before the break for kicking Greg Halford after the ball had gone out of play.
Forest took full advantage and won thanks to goals from Radoslaw Majewski and Henri Lansbury and Powell had no complaints with the decision.
"I have seen it since and it is definitely a red card," he said.
"It was the red mist for Yann. He does get a lot of knocks and is not always protected but no excuse.
"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."