MIAMI – While the extent of the injury report ultimately determines the bulk of the Miami Heat’s rotation, a subtle move by coach Erik Spoelstra at halftime of Monday night’s 108-89 loss to the Boston Celtics at TD Garden could prove telling. With the Celtics, like many recent Heat opponents, hunting favorable matchups, and with the Heat having moved to a starting lineup featuring Tyler Herro and Duncan Robinson, Spoelstra decided at the start of the third period that change was in order, making a case for the team’s defense. So no Robinson to open the third period, with Dru Smith, a guard playing under a two-way contract, in his place. “A lot of that was some of the defensive stuff that we saw in the first half and then Dru gave us that spark,” Spoelstra said, with the Heat now turning their attention to Wednesday night’s visit by the Los Angeles Lakers to Kaseya Center.