Why would the Celtics do that? For one thing it saves them the step of making a qualifying offer, which could handcuff a bit of their cap space. Even as unrestricted free agent, the Celtics still retain full Bird rights on Green, meaning they can go above and beyond the cap to sign him and have the means of competing with just about any offer.
According to Green’s agent David Falk, there will be many competing offers, or at least interested teams. In an interview with SI’s Sam Amick , Falk said he has had discussions with 12-14 teams regarding Green. Falk also told Amick that Green is 100 percent ready for next season and that he expects him to be the most sought after free agent on the market after Deron Williams, if an when Williams opts out of the last year of his contract.
There will be several teams with cap space this summer who could decide to take a run at Green. As it stands, a little more than half the teams in the league have the ability to clear space and a handful could be well under the cap including Cleveland, Brooklyn and Phoenix. Green may not be the second-best unrestricted free agent in a class that includes Kevin Garnett  and Tim Duncan  (admittedly special cases), Ersan Ilyasova and Steve Nash , but he’s in the discussion.
Green is now three years removed from his best season when he made 39 percent of his 3-pointers and averaged 16.5 points and 6.7 rebounds for the Thunder, but he will be 26 in August and should be entering his prime. Compared to other wing forwards on the market — Gerald Wallace , Grant Hill  — youth is very much on his side. Restricted free agents Ryan Anderson  and Nic Batum may have more upside, but signing them away from their teams won’t be as easy.
The Celtics stand an excellent chance to bring Green back, but they will also have competition.