Isiah Thomas was standing next to center court in TD Garden. He was aside the leprechaun logo of the Celtics, who had bedeviled his Pistons for a historically significant portion of the 1980s, until he and his mates slayed their dragon on the way to three straight NBA Finals trips. Here in the present, the Celts are back in the championship series, and Thomas, working here for NBA TV, wasn’t doing a very good job of containing his pride for one of their numbers — not that he was trying to hold anything in. Several years ago, he had established a mentor-ly relationship with Jaylen Brown, a student at Cal, where Thomas had gotten his master’s degree. Brown spent just one year on campus, and he was trying to squeeze the most from it.