You think a switch to forward could be a possibility? He could still man the point on PPs.
I think its something you keep in the back of your mind if you don't see any progression in the defensive side of his game. He certainly looks like a forward, with the way he is often the 2nd or 3rd man on the rush. He is so active in pinching as well.
But right now, i think you have to let him play out and learn his mistakes, hope for the best like it did for PK.
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Make no mistake Subban is a "big" project, who will not be ready in a year. He'll need multiple years.