The quarterback had had concussion injuries, concussion problems, or, plainly, concussions in the past. I doubt he would appreciate having his brain injury labeled an issue. An issue, as we’ve discussed before, is a topic, as in Where do the candidates stand on the issues or We have myriad issues to discuss at the meeting tonight.
Don’t refer to someone’s brain injury as an issue. It’s disrespectful——and grammatically incorrect.