Drew Cannon, a 23-year-old who was Brad Stevens' analytics guru at Butler, will be joining the Celtics, he told the Globe Monday. It's unclear exactly what Cannon's role will be on Stevens' staff, but Stevens said Cannon "will be on board in some capacity." Cannon was considered to be the first pure statistics-based hire on a college basketball staff when he joined Stevens' staff at Butler. Cannon sat baseline during the Celtics' 93-63 summer league win over Detroit at the Amway Center here. He sat next to Miami (Fla.) basketball coach Jim Larranaga and was near a slew of Celtics executives.