Summary
In his prime years, Nurkic wasn't an elite defender but was very good, although injuries have sapped some of that ability. In his last two fully-healthy seasons in '17-'18 and '18-'19, the Blazers were 7th and 16th in defense, respectively, compared to the last three seasons where they finished in the bottom five each year.

While he has never been the most explosive athlete, he could move his feet well and use his size to protect the rim. He's lost a step or two laterally, which makes it more difficult for him to defend in space, which is becoming more and more important in today's NBA. However, he does have good hands to get deflections and steals when dropping in pick-and-roll. Nurkic continues to post high steal and block rates, and rebounds very well on both ends of the court.

On offense, Nurkic has always been a low efficiency player, especially for a center. He has good hands but doesn't finish at a high rate inside, and never developed the ability to stretch the floor. However, he does set good screens and has some playmaking ability from the high post. He has developed good chemistry with Damian Lillard in pick-and-roll, and has contributed to the Blazers consistently having a really good offense.

He will be 28 as a free agent in 2022, so this could be his last best chance at a big long-term contract. The free agent market could be tough for Nurkic, as not many teams need centers and have cap space, but I think Nurkic will find a way to get a solid contract.
 
Cap Considerations
Nurkic will be an unrestricted free agent with an $18 million cap hold and full Bird rights. Since the Blazers might be operating over the salary cap, they will retain Nurkic's Bird rights, and have no restrictions on re-signing him. However, if they operate under the salary cap they may want to renounce Nurkic's cap hold in order to maximize their cap space. They also could still have some cap space after re-signing Nurkic, depending on his starting salary and what happens with their other potential free agents.

Nurkic would most likely demand a starting salary of more than the full $10.3 million Non-Taxpayer Mid-Level Exception, which means only teams with cap space will be able to sign him. If Nurkic and an over-the-cap team have mutual interest, sign-and-trade options can be explored. I wouldn't completely rule it out that Nurkic could be signed with the full MLE, especially in this market with few cap space teams.