BOSTON — All the perspective you need about this one is this: Thomas Bryant was first off the bench, with Dru Smith shortly thereafter given the keys to run the offense. No, this is not how the Miami Heat envisioned the approach to the first meeting of the season against the Boston Celtics. But with injuries decimating the rotation, that’s what they got in Monday night’s 108-89 loss at TD Garden. Unable to keep pace with Jaylen Brown or the Celtics’ 3-pointers, the Heat found themselves overmatched against a Celtics roster that was without Jrue Holiday, Kristaps Porzingis, Al Horford and Sam Hauser.