The NBA leader in offensive rating entering Sunday was on the floor at Boston’s TD Garden … and it wasn’t the visiting Wizards’ All-Star guard, Bradley Beal. The current league leader in net rating played 27 minutes on the same floor Monday … and it wasn’t All-NBA forward Jayson Tatum. The player with the NBA’s fourth-best plus/minus entering Monday night, trailing Stephen Curry and Nikola Jokiu0107 and sandwiched between Mikal Bridges and Jrue Holiday, is a key member of the Celtics’ rotation … and isn’t Jaylen Brown, Al Horford or Marcus Smart.