I don't think you have a problem AT ALL with Markelle Fultz' work ethic.
Just check out his workout routine:
https://youtu.be/uza9-7A6mNkMixed discipline training:
Swimming, Boxing, Weight Room, and of course Basketball Gym training.
You see him working on his agility, core, strength, ball handling, and shooting.
Work ethic is the LEAST of my concerns with Markelle Fultz. The guy loves to train, practice, and improve. In other words: gym rat.