What would happen in the market for knee replacement surgery if insurance companies started to cover a smaller portion of the cost of the surgery?

It would have to go down so people would have the surgery

Demand and supply will both decrease.

Explanation:

If an insurance company changed its coverage policy and started to cover less of the cost of knee replacement surgery, it would cause fewer people to seek the service of that company. That's because people would prefer to look for a health insurance company that would offer more coverage for such a delicate surgery.

As people would stop looking for this company, the demand for the services that the company offers would decrease. In contrast, with less demand, the company would offer less services, so the supply would also decrease.