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Cuneyt Cakir red cards Nani.
There are some referees who think they are big enough to make big decisions. You hope that, like professional footballers, they are also blessed with common sense.
Of course, players as well as referees make mistakes that change matches. There was an instant in the Aston Villa vs. Manchester City match on Monday.
Ciaran Clark dwelled disastrously on the ball, but Edin Dzeko kicked him in winning it from him. Mike Dean put his whistle to his mouth, thought long and hard and let play go on. Tevez scored. Match over.
Who could forget Tom Henning Ovrebo's disastrous performance in the Chelsea vs. Barcelona tie in 2009? He may have since admitted that he got things wrong, but history is written. Barcelona won the Champions League and Chelsea lost a lot of potential income.
But Tuesday night was different. While Alvaro Arbeloa writhed disgracefully on the ground in a manner reminiscent of Rivaldo in the 2002 World Cup, referee Cuneyt Cakir had at least a minute to consider his decision.
Common sense would ask whether there was any element of doubt. If there was, it would say, "Do I need to send him off, and what are the consequences if I do?"
Furthermore, during that passage of time, surely he could consult with his fourth official?
Replays over and over have shown that not only did Nani never take his eyes off the ball and therefore accidentally caught Arbeloa, but also that the Spanish player was scuffed on his arm, not battered in his ribs.
What made it even more of an injustice was that in the first half the same, Arbeloa lunged into the top of Patrice Evra's thigh with his studs up and only got a yellow card.
Let's be clear: Nani is not, and has no record of being, a dirty player. Even allowing for his past histrionics, he is more done to than doer.
But should we really be surprised at the Turkish referee's decision? He may be one of UEFA's elite, but he has now sent off no less than five players from English teams in the last four years.
While ITV's summariser Roy Keane is unlikely to get an invite to Old Trafford for the rest of his life after agreeing with the referee's decision, former senior referee Dermot Gallagher believes it was completely wrong.
At the time, Manchester United were leading 1-0 and therefore winning the tie. The decision completely changed the destiny of the match, broke 70,000 hearts and left neutrals puzzled and dismayed as to why the match could be destroyed like that.
No doubt there will be Chelsea and Manchester City fans that will gloat at the decision, but most decent-minded football fans will still wake up tomorrow shocked.
And Sir Alex was too distraught to even attend the mandatory press conference afterwards, locked in the dressing room with his players who were probably beyond despair.
One of the last poignant shots of this great manager, who had arguably suffered the greatest injustice of his long and honourable career, was of him standing on the touchline, his eyes brimming with tears.
At least Jose Mourinho sought to comfort him and believed that the better team lost.