I think players can currently go to the D-League, with their consent, for rehab assignments. No wonder that zero players have volunteered for this option, since they have much to lose if they get punked by DL guys, and little to gain. Either the injured player is a star that could rehab off the NBA bench against 2nd units, or they're role players that can rehab in practice until they're ready.
The dropoff in talent from the NBA to the DL is incredibly severe. Players with real NBA skills would be a bit out of place there. This will be the case until DL pay gets better and Euro players, or at least college players not drafted, come/stay to play there.
This is why the idea of NBA teams subsidizing the DL by paying the contracts of DL players, their 2nd round picks, makes so much sense. I think once NBA teams get their own DL affiliates permanently, as a genuine minor league, this will happen. It just hasn't evolved that far yet.
Visionaries, all of us.