I have/had similar blood clotting issues. My Dr. literally wrote the book that med schools use. A clotting issue for a guy who is young, black and semi-healthy is extremely rare! Most Dr.'s don't know what to do. Generally you see this in very old Caucasian folks. The general cure is take blood thinners forever and don't do anything "dangerous". This works for most people b/c the vast majority of patients are old as hell and aren't active anyway.
Dude either doesn't listen to his Dr.'s or his Dr.'s are idiots and don't know how to treat the condition. The vast majority of clots start in the calf. Most clots come from some trauma to a person even a strained muscle. This calf "strain" should've been immediately accompanied w/ a short dosage (a week or 2) of blood thinners as a precaution. Even if they don't come from trauma (usually hereditary condition) they most likely start in the calf. The strain+a previous condition and no treatment, I'm not surprised. Plus for many people who get blood clots high elevations (airplanes) can cause clots. He should have not been allowed to fly to the all-star game. Plus dehydration causes clots, so if he was in T.O. drinking over the break
. All these factors, I'm not really surprised.
I don't have the dudes med charts, but it doesn't have to be the end of his career. You can play sports on blood thinners. The only real risk is to your head, as the effects of concussions are more dangerous on blood thinners. Football would be a hell no, but a Dr. would most likely say you shouldn't play basketball but give the patient the option.
Another option would be to take blood thinners full time in the off season. During the season take an injection of blood thinner before or after flights. The injection wears off in 24 hours and your blood is no longer thin, so the dangers of concussion aren't any worse than anyone else. This may prevent him in playing in back to backs on the road but its doable.