If you go into a tackle with 2 feet then there is intent. When you are looking the other way trying to bring a ball down out of the air, unaware someone is coming charging into you, there is no intent. There is the difference. There wasn't a challenge by Nani, the challenge was from Arbeloa. He was the player that could see what was happening, and in a sense he ran directly into Nani, in an attempt to win the ball. There have been countless times players have been kicked/knocked in the head/ribs/chest from players trying to play over head kicks, clearing the ball or two players going for the same header. If a player now wins a header and clatters into the next guy causing him a head injury, does he get sent off? If a keeper comes out for a corner, like last night, misses the ball and punches the header of the ball in the face, should he be sent off? If a player tries to score a scissor kick and someone heads the ball away and they end up kicking them in the face, like Crouch on the weekend, should he be sent off?
For all of the above, I would say no. There is no intent to cause the opposing player injury, the move made by the 'offending' player isn't made in an aggressive manner looking to harm the other player, generally it's the unpredictable nature of the other player that causes these incidents to happen. All that's happening is that the game will become a none contact sport. There is more malicious intent in any tackle than there is in the aforementioned scenarios.