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.