Colorado Avalanche defenseman Nikita Zadorov was the key piece in the trade that sent center Ryan O’Reilly to Buffalo in the summer of 2015. To refresh your memory on the trade:
The Avalanche then turned the 2015 #31 draft pick into two — #39 (AJ Greer) and a sixth round pick in 2017 (tbd). That trade undid what the Avs had paid for veteran defenseman Brad Stuart, incidentally.
In any case, because good defensemen are so rare in the NHL, Zadorov was seen as the key piece with Grigorenko as secondary and the rest as icing.