Sounds like something Rocco Baldelli went through and ruined an otherwise promising baseball career.
I just read his story to refresh my memory about his issues. Doesn't seem to describe the same problems i have. With me, i first noticed it in grade school with the Presidential Fitness standards, where you have to run 600 yards. While i was one of the two fastest kids in sprints, with just those 600 yards, i finished at the rear, and ran them with 'the fat kid.' That was the first time i had something to compare with, endurance-related. Prior to that, half-court basketball at the park wasn't a problem. Football was fine. Baseball, no problem. But, i did play Goalie in soccer, to avoid the constant running. Later, i played youth football, and was the tailback. I was an all star short stop. But, with basketball, after one or two times down a court, i was gassed, and any skills i might have had were diminished from the fatigue.
As an adult, i dabble with tennis, and it's the same story. If hitting with a machine, i can go at a slower pace. But, as i get tired more quickly than a normal person, the skills fade and so i feel i don't improve as quickly as i should. Fewer repetitions, and less concentration because the tiredness steals from form.
When i go out to shoot hoops, just working on shooting with my 13 year old nephew, after the same five minutes, i'm panting, and he's not breathing hard at all. I keep going, though, but i know that when i'm fatigued, i'm not as focused on all the form/execution-related stuff.
Thought maybe it was a form of exercise-related asthma, but the doctor who tested me said i don't really have the symptoms of asthma, and when we tested use of an inhaler anyway, it didn't work as he prescribed it. My mother, though, around the same time, said she also has always had stamina issues, and got an inhaler, which did have a benefit. But, she was told to use it every day, while i was told to use it an hour before exercise. So, maybe i will try again, with that strategy.
There's also another thing i've only recently found out about... People—like me—get winded just from bending/being in a crouched position. It's bizarre... If i just make my bed, for example, i'll be breathing hard. Something to do with compressing the diaphragm? I forgot already...
If Kadary does have some of these issues, i hope the staff exhausts all manner of research and diagnosis to figure it out, and not just declare him 'poorly conditioned' and not a hard worker or somesuch. Hopefully, there's a fix so it doesn't compromise his career.