We have reached the end of the George Zimmerman trial and the most likely mode of injury to his face was not brought up during the trial.
Trayvon Martin was shot with a Kel Tech PF-9 9mm pistol. Presumably Zimmerman only handled the weapon awkwardly with one hand in close proximity to his face. Given this scenario, the gun's recoil would have given him a pretty good pop in the face.
The back of Zimmerman's head could have been injured only after he fired the gun and the gun recoil knocked his head back.
It is possible that Trayvon Martin never struck a blow but was only trying to get away from Zimmerman.