This is just my own opinion -
I think it's possible that GZ slipped and fell, maybe while he was following or when he caught up to TM. They've said it was quite dark, was raining, and that GZ wasn't in the best shape. He might have easily lost his footing while he was 'in pursuit', and gotten hurt. GZ might not even be certain himself, but remembered in a way that made most sense to him from his viewpoint. It was very dark, and he was likely disoriented. I would be.
Of course there are other possibilities, but this seems likely - to me.
However he got his injuries, I'm not clear about why he wouldn't have asked one of the paramedics on scene to take a look - after they had done everything possible for TM, of course. Or asked the police to get him some medical attention, gone to the emergency room - something...
If I remember correctly, he went to his own doctor the next day, or shortly afterwards.
Based on how GZ stated he was hurt, it seems as if there could have been potentially very serious injuries that would warrant at least an emergency room visit. I'd think LE would insist he get immediate medical attention, just for liability reasons.
Just my opinion, and I apologize if this has already been mentioned.