The injuries are not alleged. Zimmerman's claim that they were
inflicted by Martin is what is alleged.
The police arrived, Zimmerman confessed to shooting Martin and, as I
understand it, the injuries to Zimmerman were fresh at the time.
I don't think you need a witness to make a case of self defense. That
said though, if Zimmerman instigated the altercation by pursuing Martin
and challenging or harassing him in any way, then I suspect his "stand
your ground" defense will go out the window and he'll be found guilty.
Just my guess.
The witnesses so far seem to indicate that Zimmerman started the
altercation, that the screaming seemed to be coming from Martin, that
Zimmerman was on top of Martin after the gunshot, et cetera.
I have a feeling Zimmerman is going to have to take the stand. That
should be interesting.