But if Durant opts ut and testing the market, GSW can offer him a 5-year contract worth an estimated $221 million.

https://knicksfilmschool.com/2018/10/26/how-do-the-knicks-create-enough-cap-space-to-sign-kevin-durant/
Personally, I think they should refuse to offer Kevin Durant an multi-year max contract.
I mean, we're talking about a guy who won't even play basketball again until he's 32 at the start of 2020-21 season. And not only that, Achilles injuries really require 2 years of healing time before you even know how well someone's recovered. The first year back is spent just trying to get your body acclimated to playing again. Your range and strength during that first year back are nowhere near what they used to be and then there's the whole psychological and mental components to the recovery as well.
So we're really looking at the 2021/22 season (3 seasons from now) before we truly know if Durant will ever be great again. He'll be 34 y/o at that point. 34! And that's if he even manages to stay healthy during that time which is far from guaranteed.
It would cost the Warriors the rest of Curry's, Klay's, (if he re-signs with GSW) & Dray's prime years to waste over $120M over the next 3 years on Durant who won't immediately return to his previous level & help the team win a title during that time. 3 years!
Like I said in the beginning, Durant has a $31.5M player option for next season that he's free to opt into. No other team will be willing to give him anything close to a long-term max contract this summer, so he's extremely lucky to have that massive player option to fall back on. That more than makes up for his injury right now - he wouldn't get a contract this summer if he was a free agent now. He'll actually get paid $31.5M next season just to rehab - that's more than enough compensation for him getting hurt.