Depends. If the referee thinks the touch on the ball was the striker it's a yellow. If he thinks Buffon took the man and then the ball it's a red.
Because he's completely through and it's just a one on one (which is a clear goal scoring opportunity) if Buffon's fouled him and then taken the ball away. If the striker has nicked the ball away to the defender it's a yellow as he's stopped it being a goal scoring opportunity by his own doing.
But the striker had no control of the ball - it was an underhit backpass that he chased down. The ball was behind him when he was taken down. Even if he had nicked the ball away from the keeper it would have probably gone away from the goal.
He's obviously interpreted(imagined) it as the striker nicking the ball away from the goalkeeper towards the defender so it doesn't matter. Imagine if a striker was running onto an under hit back pass and a defender went through the striker and then got the ball to prevent a one on one with the keeper. It would be a red. Even more so if it's the actual keeper.
For a red card to be given for denying a clear goalscoring opportunity, the attacker must have been able to reach the ball and have a clear chance to score. Not so in this case.
