Doc Rivers has always pulled Kevin Garnett and Paul Pierce together, and in another age Ray Allen, to schedule a season-ending meeting. Last night the Celtics coach gestured his two oldest vets over to the side, and made preliminary plans to talk about the future. But never has the future appeared so grainy in this aging era of Celtics basketball. Pierce has said he wants to play beyond this year, though that could leave him in Clippers garb as easily as the green uniform he’s worn his entire professional life. Garnett said last night that the opportunity to play with Pierce was a big reason he became a Celtic, and it could now be a deciding factor in whether he remains, even if he’s under contract for two more years. So maybe last night’s 88-80 loss to New York, complete with a failed fourth-quarter comeback, was nothing more than the death rattle for a great era. Danny Ainge, Rivers, Pierce and Garnett will make those decisions this summer. The Knicks, after taking a 3-0 lead in the Eastern Conference quarterfinals, ultimately needed six games to eliminate the Celtics. But despite a 24-2 fourth-quarter run, which cut a 26-point Knicks lead down to four (77-73) with 4:06 left, Pierce especially seemed to take another step toward calcification. He appeared to turn to dust while grasping for entry passes. He was unable to exploit matchups against opponents like Raymond Felton and Jason Kidd — small guards he would have toasted in another age.