As much as he has his teammates to thank, he made sure the US won that last relay. His leg gave Lezak a lead that he was able to keep over Sullivan. If it wasn't for Phelps, the US probably wouldn't have won that race. That said, Lezak is the one that made sure 8 golds was possible. His leg in the 4x100 was absolutely phenomenal.
Phelps is a genetic freak. The fact that he was able to do what he's done is not surprising at all. He won 6 in Athens and the one he lost is now one of his best events.