Graham Poll in the Daily Mail says the sending off was correct. He says going head to head with each other should attract a caution to each player. He goes on to say the withdrawal of the head by Nasri, and the flick that followed it constitutes a head butt and a red card. No arguments there, and pointless appealing IMO.
The same article says Bassong was cautioned for the tackle. So by the same logic the head posturing should have been a second yellow card for him, and dismissal also. That is the injustice Mancini is rightly annoyed about. Blinkered Poll fails to address this.
Addressing other points discussed in this thread, Nasri should not have reacted, and Mancini needs to know he has players he can trust on the pitch. Hence Balo's limited deployment this season. Nasri deserves to face club discipline if he has broken the high standards of behaviour expected of our players, because stupidity like that could easily cost us points.