"Playing Basketball" Isn't that fatiguing.
Competing at an NBA level is - that means not taking plays off, regular contact, holding, hacking, shoving matches, and battling nearly every minute you are on the floor.
Backing down and being backed down in the post night-in-night-out is comparable to an intense session of weight lifting, with added cardio to boot.
While dealing with muscle fatigue in back to back (to back) games, players will likely be less explosive and mobile, regardless of how in-shape they appear aesthetically.
I fully expect play around the league to look more crisp and polished in the coming weeks as player's bodies adjust. Until then, certain teams and individuals will suffer due to the shortened schedule.
For an athlete, preparation is key.
The fact is, teams just haven't had the prep time, both mentally and physically, that is required to play at this level under these scheduling circumstances.
Players will adjust - getting stronger, faster, more explosive - as the season goes on. However, each night they go in there and battle, they are taking a toll on their bodies. To gain strength, endurance, and explosiveness, your body must build stronger neuromuscular pathways as well as create change in the physical muscule tissue.
For the latter, a change takes place in the muscle both during and after exercise. There is wear and demand on the muscles from exercise, and a degree of breakdown. This breakdown is extenuated predominantly after the exercise itself, and the muscle continues to tear itself apart in a hope to re-build (much like many NBA franchises).
This process may be fairly short, perhaps 12-24 hours, and may involve little or no soreness. However, during times of extreme strain, the muscles may undergo trauma due to fatigue and overuse (such as competing in an Iron Man) to a degree that they may take several weeks to repair fully.
In the case of a back to back, a player isn't giving themselves proper time to rest and re-build, and their explosiveness and energy reserves will be affected as a result.
Despite this, the human body is an amazing thing. Many players will adapt and thrive during this season, getting into some of the best shape of their lives. An off season of rest for them will be welcome, but they should be more or less okay.
Other players, however, will take longer than a few weeks to adapt to the increased demand of output. This will result in cramps, injuries, limited minutes, and DNPs due to a variety of reasons.
Who will thrive and who will succumb to the fatigue? It remains to be seen.