Concussions can have a major effect if not treated corectly. in some cases it can be hard to diagnose. i havnt read any of the links on this thread so all i can say is from experience. i watched and boy did that look bad, if you noticed, as soon as he was hit he was out and then came to and that is very bad. as far as after effects, Post Concussion Syndrome, something i suffer from, is also no joke as it can totally change a person.
Keep in mind also, there have been a few, if not many, case where people have been hit by a foul baseball or hockey puck during a game and feel fine only to go home and be in bad shape, or in one case that i can remember, die. so, again, i dont know the case, but, from what i know of his possible injury, he is far from in the clear.