You must have seen his demeanour in the games? He hangs his head after almost every opponent basket, he hands his head after almost every miss or turnover. These kind of things are nothing to do with fatigue or his illness, if you assume him not bothering to get up and down the court is.
He's big on loyalty, I think he'll give this situation every chance he can to develop positively but he's also an ambitious professional, if this thing is going nowhere, no one would blame him for making noises about leaving. That said, we're all PO'd at this situation and he might afford a bit of loyalty to his teammates by at least pretending he has his head in the game.
If his illness is persisting to the point where he can't get up and down the court or show on pick and rolls then Corbin needs to sit him down.