More than half of high school athletes take protein supplements and percentages could increase in college and in professional sports. The NFL website strongly suggests that protein supplements can be helpful. Whey protein supplements that are quickly absorbed can have very adverse effects. Whey protein isolates are very quickly absorbed and can have very adverse effects. Most whey protein supplements contain at least some whey protein isolates. Whey protein isolate would contain only whey protein isolates but supplements labeled as whey protein usually are partly whey protein isolates.
Head injuries may not be the only problems of athletes are facing in terms of cognitive abilities. Whey protein isolates can negatively affect cognitive abilities. Any supplement formulated for quick absorption can have negative effects with whey protein isolate being one such supplement.
Casein is a much safer protein supplement but I am not recommending that athletes switch to casein from whey protein as taking supplements is full of pitfalls. The calcium phosphate in casein, for example, could decrease absorption of iron. Athletes who hold they need more protein should eat more high protein meals.