The Boston Celtics’ disastrous 2018-19 season came to an end on a Wednesday in May. The dejected squad dressed quietly in a Milwaukee locker room, then flew back to Boston the next morning. A few players talked to the media, but they didn’t have any answers. Nobody really did. The next morning, Celtics assistant coach Tony Dobbins got a text: Could he get to the practice facility? Jaylen Brown wanted to get in the gym.