Not a medical professional, but that could easily be caused by something like a subdural hematoma.

I'm reminded of a talk at a science fiction convention years ago mostly targeted at writers (and done by a physician) - "Maim 'em Right." Basic message was that when writing injuries, make them realistic. If someone takes a blow to the head that knocks them unconscious for a significant amount of time, they're not resting for a day then getting up and going on to lots of exertion and winning the day. They're suffering from concussion, maybe subdural hematoma/brain bleed, etc.

Also discussion of TBI, including things like shaken baby syndrome - in the skull, the brain has a consistency somewhat more durable than thick jello. Get one of those brain jello molds available around Halloween, make a batch, then instead of unmolding it keep it sealed and give it a good shaking. THEN unmold it and see how much damage there's been.