That was never a red. The criterion for a red card is clear: excessive force. FIFA defines excessive force as follows:
"Using 'excessive force' means that a player has far exceeded the necessary use of force and is in danger of injuring his opponent."
Perhaps Nani was in danger of injuring Arbeloa, though not exceptionally so. It doesn't matter, though. He didn't meet the first criterion of excessive force, which is the use of far more force than is necessary. The word for what Nani did is reckless. Reckless means yellow. The referee messed up.