I agree that the "only" atlas adjusting is not the best protocol..
Something along the spine or any other joint... could be out from a previous injury/accident/sports or the concussion..

I'm glad my DC did atlas as well as whole body adjusting.. and used a mix of modalities like ultrasound, IFc stim, low level Laser therapy,massage..
My main problem was chronic RSI/myofascial pain.. but concussion/LOC 30+ yrs ago.

The strained neck muscles will tend to pull the alignment back out, that's why treating the muscles is an important part of the puzzle.