It was a strange decision tbh. They should have let the year play out, see if he could remain healthy, and then worst case scenario, match whatever offer sheet he received from another team (he wasn't going to play out his 5th year w/ no guarantee and become an UFA). That 5 year max would have still been there if all had worked out.
I get that it is good business to lock up your players early and make them happy. But Porter Jr. had only recently broken out, and obviously his injury history should have been taken into consideration.
Hopefully Porter Jr, Murray, and the rest of the Nuggets team can get and stay healthy in the future. They are a fun team and very easy to root for.