I think Green's issues are multi-fold. First, his confidence has been completely shattered, and he is way too tentative in how he goes about his job. Only way to fix that is by getting in his head to adjust his mindset, and experience success on the field. Second, his injury history has made him a physically lesser player. The combination of lower leg and hip injuries have robbed him of explosion and athleticism. His inability to adjust to lateral movement, as well as the inability to shuffle/slide to stop a wide nine rush are products of this. His only chance at this point is to be a phone booth type of player as a guard, but I'm not sure that he's physical enough to succeed at that type of role.
His issues with hydration are likely not related to conditioning. The team training staff should have a workup on him to indicate whether he has issues with retaining water (electrolyte issue? potassium?). Not sure how deeply this has been looked at, but, if they did provide him with a diet/hydration plan to follow, and he doesn't religiously follow it, well, there's your cause.
Yes, they do need to move on from him, but it's not as simple as to just say that he sucks. There are reasons for it - some lie with him, and others lie on the fate that has befallen him.