I'm not sure what your point here is. Are you contending that it's not possible for Zimmerman to have lied immediately?
No, I'm contending that Zimmerman really didn't have time to make up a whole ficticious story and that since the evidence supports what he said it's likely he was telling the truth.
No, the logical conclusion is that Zimmerman struck his head on something. It would take a leap of logic to assume Martin bashed Zimmerman's head on the pavement because there's no evidence that actually occurred.
Do you have evidence that it happened other than the way Zimmerman said it happened and which the evidence corroborates?