15 or 20 years ago that would have been a superb challenge from Sandro, perfectly timed, took the ball, excellent. However this isn't 15 or 20 years ago, it's 2013 and I'm sorry but the pundits who agreed it was a fine challenge are simply demonstrating that they aren't really up to date with modern football. Yes, he played the ball first, but he did so in full knowledge that he was going to slide straight through the back of Nasri and clean him out. The "tackle from behind" rule interpretation was introduced to prevent injuries caused from just such a challenge. It's a foul, wherever it occurs on the pitch.
It's also worth noting that, whilst Sandro did indeed play the ball, he didn't clear the ball away from the danger area, he simply poked it a couple of foot ahead of Nasri. If he hadn't then continued his clide and brought Nasri down then he would still have had the opportunity to reach the ball and have a shot. So the fact he took Nasri out with the follow through has actually prevented a goalscoring opportunity.