Being knocked out is by definition brain damage, it’s a concussion serious enough to have caused the brain to essentially take a vacation. I think the limit is about two minutes for a head strike, if your character is down longer than that, then they’re dead.
The best part is that this doesn’t actually require a lot of force. The brain is basically four pounds of warm butter suspended in fluid, it doesn’t take much knocking about before it decides it wants to be elsewhere.
This is why even high school level football claims casualties from time to time and that’s with protective gear.