I've thought about this off and on, actually, though not specifically related to head injuries--just how they stay capable of fighting after knocking each other out all the time in general. (I've been thinking about all fighting games, not just SF, though it started when I actually ran the old SF tabletop RPG many, many years ago.) My take on it is this:

-They strike with full force--not always to kill, certainly, though some do. Always to knock out, though--their honor, both giving and receiving, would demand no less. If you're not giving it your all what's the point? You'll just lose to the ones who are, and shame yourself and them in the bargain.

-They do, however, manage to be in good shape for the next round. We can skip the 2/3 rounds stuff here, of course, but still, injury and whatnot in the real world takes far more time to heal than it does in the SF (or KOF, or wherever) universe. From this I infer that all martial artists on this level have had specific training in techniques necessary to heal quickly, as a ki ability. Ordinary people would not have that benefit, and there are even more advanced techniques for healing available (in the SF RPG, it was called chi kung healing, but every style surely has its own name) and, even if the greater culture isn't aware of those techniques the martial artist subculture would be. Soon as they regain consciousness they can meditate and focus, whatever their sensei taught them. I see no reason whatever why we can't say this ki healing can't heal up the little hits that build up to traumatic brain injury as well. This sort of thing happens in fictional universes -constantly-.

-TLDR: Option 2, above. =)

These are educated guesses, of course, but they preserve the balance of explaining away real world injury and game world necessity. They're internally consistent with the fictional universe's rules and fit neatly into what we've already seen in both this series and the canon SF storyline.

As for the bigger question, what to include in the series concerning this issue, I'd recommend possibly making an OOC note to show you're thinking about this and you're not glossing over real-world traumatic brain injury, and perhaps a quick flashback scene for Sakura, showing how Gryph taught her to recover quickly. If she didn't already figure it out!

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Fearless creatures, we all learn to fight the Reaper
Can't defeat Her, so instead I'll have to be Her