Summary
Dinwiddie's ACL tear certainly complicates his free agency, but I think he'll still get a sizeable contract. Despite never having great shooting numbers, Dinwiddie is a solid on-ball creator that can operate as a lead ball-handler or secondary creator next to a ball-handling wing or another point guard. On defense, Dinwiddie doesn't shine, but doesn't bleed points on that end either. He's an intelligent player. He has good footwork to finish creatively at the rim - Dinwiddie shot 64% at the rim in '18-'19, which was the 92nd percentile for his position, according to Cleaning the Glass.

There aren't many teams in need of a starting point guard, but in today's NBA you can play him at either guard position, especially if your point guard is big enough to guard opponent's shooting guards. Dinwiddie also has enough size to guard some wings if necessary.
 
Cap Considerations
Dinwiddie has a $12.3 million player option for 2021-2022 that he is likely to decline. If Dinwiddie does opt out, he will be an unrestricted free agent with a $17.2 million cap hold and full Bird rights. Since the Nets will be operating over the salary cap, they will retain Dinwiddie's Bird rights, and have no restrictions on re-signing him. However, the luxury tax will certainly be a concern, as re-signing Dinwiddie could cost them a lot of money in luxury tax penalties. Therefore, other teams will have a legitimate shot at signing Dinwiddie away from Brooklyn without having to give him a huge offer.
 
Dinwiddie would certainly demand a starting salary of more than the full $9.5 million Non-Taxpayer Mid-Level Exception, which means only teams with cap space will be able to sign him. If Dinwiddie and an over-the-cap team have mutual interest, sign-and-trade options can be explored.