Obi-Wan ducked, narrowly avoiding Sanzo's foot by inches. He knew he couldn't allow himself to become disoriented by a blow to the head -- it could throw him off balance long enough for the other man to fire another bullet, and actually not miss.

At that same moment, he heard Goku's pleas, and he was glad he at least hadn't fallen for this game. Without wasting a second, the Jedi Knight leaped towards Sanzo, and with fluid precision, moved to grab the arm holding the gun. He had to disarm him.

If he was busy fighting for his life, then there would be little he could do for themselves, much less for Qui-Gon and the boy.