As a restricted free agent, Mills is in a situation similar to Wilson Chandler's, which has been well documented. Because it's March 1st, Mills can no longer sign an offer sheet with another team, so if he hopes to play in the NBA this season, he'd only be able to sign with the Trail Blazers.
Portland has a full roster after signing Joel Przybilla, so it's unclear if they're interested in bringing Mills aboard. Unlike Chandler, Mills wouldn't necessarily be eligible for unrestricted free agency this summer if he just signed a rest-of-season deal, so that seems like a possibility. If the Blazers aren't interested in adding the 23-year-old to this year's roster, they could renounce him, allowing him to sign elsewhere. The 6'0" point guard could also sit out the rest of the season and become a restricted free agent again in the offseason.