Benitez's side beat United 1-0 at Old Trafford, with full-back Rafael sent off for kicking his compatriot in the second half.
The Chelsea man was laughing as he lay on the floor, but has since said that he was smiling at the jeers he was receiving from United fans at the time.
However, Benitez claimed that the issue was frivolous and that it had detracted from the real problem of poor challenges that need to be outlawed in the game.
"I don't think it is right. Luiz was laughing because the fans were abusing him," Benitez is quoted as saying in The Express.
"The red card came after that. But we are talking about this rather than attempting to eradicate the bad tackles that you can see every week. There are tackles every single game that can break ankles and legs and nothing happens about them.
"But try to protect the ball and have a smile on your face and it's the end of the world."
Chelsea host Tottenham Hotspur this evening.