Even though Juan Martin Del Potro's name is still on the entry list for the US Open, the reigning champ has no idea when he will returns to the tennis courts. This is not good news because no player will defend a title without some warm-up tournaments, especially, a slam.
Del Potro will likely withdraw from the tournament citing not enough matches, even if he is to be ready to hit some balls by then. This is just a guesstimate, but maybe he will come back to play just to get some matches in.
It would be strange since no reigning champ wants to exit first round (high probability) but the Argentine could love tennis enough to bypass that thought altogether.