It only has to be considered a contributing factor through expert opinion, not conclusive study I would of thought. If the consulting neurologists believe that concussion was a contributing factor to his current symptoms, given this is a civil claim, that would probably be sufficient to then assess whether the AFL was negligent in dealing with it.
It also has to be shown that their management was negligent in light of knowledge at the time. Research into the long term effects of concussion has skyrocketed of late (largely because of the NFL), so it is entirely feasible that the AFL acted in line with the knowledge at the time.
Will be interesting to see the outcome, but one suspects if this case is successful there will be several subsequent ones to follow.