THAT CONTRAST IS encapsulated in a pair of moments — one last season and one this — with striking similarities. On a Saturday in January, the Celtics trailed the Orlando Magic by two points with 2.9 seconds left. Jayson Tatum curled to the corner, took an inbounds pass from Gordon Hayward and missed a game-tying shot. Irving was furious. He had words for Celtics coach Brad Stevens in the huddle, then for Hayward on the court after the final buzzer had sounded. In his postgame interview in the locker room, he proceeded to take his latest round of veiled shots at the team’s young players.