Why Bruce Cassidy is switching up his defensive pairings

The first quarter of any given season often provides coaches unique opportunities with their lineup structures. And surely, Bruce Cassidy used his philosophy for his defensive pairings and forward trios outside of the top line of Brad Marchand, Patrice Bergeron and David Pastrnak.

Two days removed from an ugly 4-0 loss to the red-hot Calgary Flames, Cassidy tweaked his top two blue-line pairings during Tuesday’s practice. Boston’s sixth-year bench boss broke up the struggling middle pair of Matt Grzelcyk and Brandon Carlo and the top duo featuring Derek Forbort and Charlie McAvoy. Forbort and Grzelcyk swapped their left-line assignments while Cassidy kept the Mike Reilly-Jakub Zboril pair…