The story does not need Zoro to fight Kaido right
now:
- Yamato is currently stalling Kaido.
- Thus, Zoro is not needed to fulfil the role of stalling Kaido.
- Yamato stated that she would hold back Kaido until Luffy returns.
- She'll probably succeed at that.
- Yamato and Kaido's battle needs privacy due to their personal relationship.
- It's unlikely that Zoro will interfere.
As such, something will be needed to occupy Zoro in the interim. Given that Zoro is on a time limit when he recovers, he'll likely immediately jump to fighting, and after Kaido, King is the most suitable opponent.
While King can be used to stall Zoro, I don't think he will be
needed to fill this role. Chapter 1017 featured Chopper and Miyagi decide to administer the ultra regenerative medicine to Zoro. The effects of the medicine and Zoro's actual recovery can take as long as Oda needs it to. There is no real guarantee that Zoro will be fully recovered within the next 5 chapters, and to be honest I do not expect him to be. It seems that Chopper's role for the rest of the arc will be Zoro's treatment:
So I don't really expect this to be completed quickly.
Furthermore, Zoro and Luffy were shown receiving treatment in the same chapter:
If this was deliberate (and if it continues), then Zoro will return to action at the same time as Luffy (this may be after Yamato and/or King have been defeated).
If Oda takes the long route with Zoro's recovery, then King will not be needed (and may be unable) to stall Zoro.