I have been in a similar situation due to so-called mild traumatic brain injury.

My experience is that first you see a doctor who typically sees this sort of patient a few days after the injury has occurred. They will say that is will get better after a few days.

Then when it doesn't you move on to see a more specialist doctor, way a neurologist, who will typically see a patient with brain injuries, a few months after the patient was injured. They will say that it should get better after a few months.

Finally, you will see a rehabilitation specialist who will typically see patients only after perhaps a year or so. That specialist told me that it would get better in 1 to 3 years.

So, do they really know?

As far as I know, they don't do any systematic follow-up. So when a patient doesn't come back, they might assume that he or she has recovered. In reality they may merely have moved on to a different part of the health service when one part hasn't done them any good. If they typically see injuries a certain length of time after the patient was injured them may naturally assume that that time-span is the natural one for that particular type of injury and advise the patent accordingly.

I think that doctors can come to incorrect conclusions unless results are monitorred properly and you only get that type of information in scientific studies that are peer-reviewed.

Regarding who to believe: I think that to err on the side of caution would be good. This type of condition can go on for years and years and be a terrible influence on someone's life. It there is any doubt it isn't worth risking it. Take up a different sport. There are plenty of sports that have little risk of taking a blow to the head.