The Herald’s Steve Bulpett was first to report the story.
This is very encouraging news for the Celtics and Danny Ainge as he attempts to bring back the core of the team for another run. The veteran players had all expressed support for Rivers while he took some time to consider his options.
It also allows the Celtics a degree of stability in what has been an eventful few weeks since their loss in Game 7 of the NBA finals  against the Lakers.
In addition to Pierce’s decision, Tom Thibodeau  was introduced as the head coach of the Bulls ending what had been a very successful three-year relationship between Rivers and his assistant. Also, Kendrick Perkins  told reporters Tuesday that he would have surgery to repair a torn Anterior Cruciate Ligament in his right knee and would probably be out until late December or January.
What isn’t known yet is whether Rivers would consider extending his contract beyond next season. But with the start of free agency Thursday, his return eliminates some of the guesswork for potential free agent targets and also increases the likelihood that veteran players who miss out on big contracts would sign with the Celtics.