There are a million and one things that will have irked Manchester United fans following their abysmal performance in the 4-0 defeat to Chelsea yesterday, but one decision from the referee has had social media up in arms.Chelsea defender David Luiz was fortunate to escape a red card for a high foot on Manchester United midfielder Marouane Fellaini and Twitter was unanimous in the belief that the Brazilian should have been given his marching orders - although it's unlikely it would have changed the course of the game, given how poor United were.Sky Sports pundit Graeme Souness was amongst those who felt Luiz should have been sent off, as he suggested referee Martin Atkinson bottled the decision to dismiss the Chelsea man."That's not a difficult decision to make," Souness said. "If you look at where the referee is. He's getting a full view of it. Does he not see six studs showing there?"Does he not see that? He says "I've seen something, so I'm going to give him a yellow card." By the shape of David Luiz going for the ball, that is extremely high, showing six studs... it's a red card. Martin Atkinson has completely bottled it."Understandably, United fans felt extremely aggrieved with the decision that went against them and they took to social media to vent their anger.Here are some of the best responses...