One incident doesn't change that fact.
As well, this "4th liner" is a 1st round draft pick (12th overall), 70th percentile in skating speed. He is 6'5" - a big guy taking longer to develop as is often the case with big forwards.
Rielly's gap control on the puck carrier McBain seemed reasonable. As McBain decelerated and cut to his left, Rielly stayed with him. But But was following up the play, building up speed to sling shot past Rielly.
I do not think that is as big of a defensive crime as it is being made out to be. In my opinion, the vast majority of NHL dmen would get beat on that play.