He wont retire undefeated. He will get his L at some point, either hang around for one fight to long or sooner. I think Zab showed that he is beatable. If someone can keep a high work rate with presure against him, it would give him problems. Zab showed in the first 4 rounds that with speed and a good work rate you can beat him. But the problem is keeping that work rate up for the full 12.