The point isnt whether the injuries were there or who caused them, the question is who was acting in self defense? It could be argued that Trayvon was, because when asked "whats the problem", Mr Z reached for something on his right side. As it turns out the only thing to reach for on Mr Zs right side was his gun. I would be arguing that if I were the prosecution.
The 2012 Florida Statutes
Title XLVI
CRIMES Chapter 776
JUSTIFIABLE USE OF FORCE View Entire Chapter
776.012 
Use of force in defense of person.—A person is justified in using force, except deadly force, against another when and to the extent that the person reasonably believes that such conduct is necessary to defend himself or herself or another against the other’s imminent use of unlawful force. However, a person is justified in the use of deadly force and does not have a duty to retreat if:
(1) He or she reasonably believes that such force is necessary to prevent imminent death or great bodily harm to himself or herself or another or to prevent the imminent commission of a forcible felony
Trayvon Martin had A LEGAL RIGHT to defend himself (not using deadly force) if he had reason to believed Zimmerman was about to attack him. He had every reason to believe Zimmerman meant to attack him. Zimmerman was following him in dark, deserted place. When Martin ran, Zimmerman ran after him. Martin had every right to turn on Zimmerman and use force against him. Martin had no idea who this guy was, not that he was neighborhood watch, nothing except some wierdo was following him and meant to do him harm.
Zimmerman had a right to defend himself if attacked, but NOT TO USE DEADLY FORCE. His life was not in imminent danger. He knew the police were minutes away. He know he had a gun, he was not being so beaten, if at all, that he needed to kill someone to save his life.
I do hope the jury will realize this. As a woman, if I were in Martin's position, with someone following and then chasing me in a dark, deserted place, who ran after me when i started to run away, if I were capable, I would turn and attack that man. Of course I would. Anyone would. And in Florida, if not everywhere, I would have a right to if I thought I was at risk to be attacked. And Martin had every reason to believe he was.