PHILADELPHIA — Preston Mattingly walked into the Phillies’ clubhouse at about 6 p.m. on Oct. 15. With an 8-3 win over the Braves, his team had just clinched a spot in the National League Championship Series. Few expected them to be here. The postgame celebration was bound to be rowdy and Mattingly, the Phillies’ director of player development, was excited to be a part of it. But something was nagging at him. Two days earlier, Phillies minor leaguer Corey Phelan passed away from T-cell acute lymphoblastic leukemia, a very rare and aggressive type of cancer. Phelan was only 20. Mattingly had sta…
