It would still be a zanpakuto.. but like one of the ones shinigami trainees have that dont have a soul in them, or something.. or one that just isnt awakened.. like that.. kinda
Or it could be engetsu still, its just hes not there anymore.. because he lost his zanpakuto instead of his powers.. Along those lines.. Its a possibility, anyway.. i just want to see what others think about it. It could be the reason why he didnt release.. besides that he is still too weakened to. He didnt even go shikai.. I had always thought that to use a zans power you have to at least be in shikai mode.. which he did it while he wasnt in shikai.. I believe, anyway.. Although, aizens shikai powers make me wonder a bit about that.. but his is so hax and works completely different than isshin/ichigos..
Well, think of it more like its engetsu still, but without his soul in it.. That or hes dormant or something similar..