Luiz was seen laughing after he was floored by the challenge, and United boss Sir Alex Ferguson accused Luiz of going down like a "dying swan."
"I'm aware that David Luiz is one of the best players in the Premier League by miles, without doubt," he said.
"We are talking about Luiz laughing because the fans were abusing him and the red card was after that.
"We are giving importance to this but it is not as important as other things. We are talking about this instead of trying to eradicate the bad tackles you can see every week.