Due to the sheer amount of time Lucci matched Zoro in CQC and (advanced) armament haki, and stalled him. Plus minor bruises here and there for Zoro. It cannot be a neg diff or even a straight up low diff.
Still, Zoro one shotted him without bandana, and if spoilers are correct without KoH too (unsure of whether conqueror's was used tho).
Considering everything we know so far, I say low-mid diff (leaning towards low than mid)