Here is a case of an 18-year-old football player with no documented history of concussion, with deposits of tau protein in the brain. Tau normally is a cushion between neurons but becomes a scavenger/free-radical following repetitive head trauma, even repetitive sub-concussive blows. Here is a great article on CTE, although it is quite technical…
In short, there is no amount of damage that is good for a brain, diagnosed concussions or not…