I have been living with psoriatic arthritis since my late 20s. I don’t know if this is the “type” of arthritis he has but I can tell you that there are ways to mitigate it but on certain days it just feels like (in my case) my feet or ankles won’t even support my weight.
Arthritis is mostly thought of as an old person’s illness. But it’s not and can be painful as hell. If it’s in his ankles, toes and knees like it is for me, it’s more a matter of pain than strength. It feels like you can’t rely on it so you tend to limp, or favor the hurting area.
I can only imagine how that would affect workouts, practices, and actual game play.
Fortunately, there has been a lot of research and new remedies for it. Unfortunately, if it’s a case where bones have been altered, it would take surgery to correct or you just use medicine to slow or stop the progress.
Bottom line is that we need to have two backs. But it really explains to me why he would be on fire one week and look average on others. It seems to work that way.