Say that he got hit with the first bit of logs but the ones that would have killed him missed him, leaving him injured but alive? Would Zira have gone and tried to help nurse him back to health, rewarding him for trying to kill Simba, or would she urge the group to go on and attack Simba since he was injured, possibly leaving Nuka to die of his injuries in the meantime?
(BTW, if she did suggest that they just leave Nuka to die so that they could go after Simba, it's possible that it may have had caused the same effect as when she told Vitani that if she didn't fight that she would be killed too. Leaving Nuka after trying so hard like that would really show the Outlanders what Zira really was.)