At the end of it, the scorer was in the more dangerous position. When Senderos was having to make the choice, it was the guy running into the box unmarked and seemingly heading for the centre of the box. As soon as attacker peeled away to the left, Big Phil tried to get back but he made the right choice at the time.
I blame Song for not picking him up in the first place, far rather Senderos hadn't been forced to make that choice.