In what way are his injuries inconsistant? For the most part having your head banged against flat concrete is a crushing injury, not a tearing one, so you could actually claim that any lacerations are inconsistant with it. However, having noted that it's unlikely that there would be zero sheering to such an action, so minor laceration could easily occur, exactly what we see. What is unlikely is major lacerating of the scalp because that would require a sharp object to tear the skin.
That he was walking fairly easily after 30 minutes? I used to referee Rugby Union and I have seem rugby players severly concussed during a game, almost unable to stand up by themselves, but who after being checked out by St John (equiv to volunteer Paramedics) and given 30 mins rest were pretty close to normal. How then is seeing someone walking about 30 minutes later any indication of what they were like immediately after the incident?