I think Nail sticking with Sedin there kind of threw everyone off on that play. Classically you have the wingers covering the points while the center takes the high forward (in this case, Kassian). The defensemen are usually working with the center in containing the forwards. In this case Yakupov took Whitney's assignment (forward along the left side) and Whitney was a bit stuck in limbo figuring out what to do.
Look how ridiculous that picture is. Hemsky (RW) is busy covering Yakupov's post (Vancouver RD), Whitney is stuck in limbo, and Gagner is watching as Kassian breaks in. Sedin had 2 beautiful passing opportunities there: 1) Kassian, and 2) the completely uncovered Vancouver LD who Hemsky abandoned to cover Yakupov's man (not in the picture).
Proper adjustment would have been having Whitney take the darting Kassian, while Gagner gets in the passing lane to cover the LD - but that was unrealistic for such a fast-developing play. In essence the expectation is that the players follow through on their original assignments (Gagner checks Kassian, Whitney gets suck in limbo)
In essence, the goal is the fault of all 3 players - Yakupov, Whitney and Gagner.
Yakupov - broke with systems and chased a forward behind the net and along the boards, throwing off Whitney.
Whitney - for not adjusting to the switch and instead bone-headedly chasing Sedin+Yakupov to the left boards, which wouldn't have been TOO bad of a play if Gagner hadn't screwed up
Gagner - responsible for the high forward (gearing for one-timer or crashing net). Clearly did nothing to impede Kassian's progress to the net or his shot.
Overall blame breakdown IMO would be 25% Yakupov, 50% Gagner, 25% Whitney.