It goes something like this.
Paul Pierce is waived by June 30. Danny Ainge has the entire month of July to look for upgrades to the roster. If he has the pieces in place, Pierce agrees to come back for the minimum and KG doesn't retire. If Ainge can't make any deals that make them think it is worth it, then KG announces his retirement on August 1 and Pierce takes his talents to wherever he feels comfortable finishing out his career.
Find a solid player willing to sign for the MLE (possibly now available if waiving Pierce puts the team far enough under the apron). Put together a pupu platter of some combination of Lee, Crawford, Melo, and the unguaranteed Chinese Three for a team looking for cap relief.
Get lucky and you can find two quality veterans who are rotation-worthy on a solid playoff team and add them to Rondo, Green, Bradley, Sullinger, Terry, and Bass.
Imagine for example if the Celtics signed Samuel Dalembert for the full MLE and traded Crawford and unguaranteed contracts to Toronto for Landry Fields, giving the Celtics someone who can be that tall SG off the bench behind Bradley that some people crave. I'm not sure how the the Raptors feel about Fields (an overpaid player who I don't think is all that great), but they have DeMar DeRozan and Rudy Gay starting with SG/SF Terrence Ross behind them as a guy they reportedly consider untouchable. If Bryan Coangelo is on his way out, then a new regime may want to clear one of his biggest mistakes from the books.