We have an overcrowded attack and fewer games next season. I mention Hernandez as his gametime will likely mean Rooney/Welbeck go out wide in those games potentially, instead of Januzaj - it's all related.
Again, we currently have Valencia, Nani, Young, Mata, Kagawa, Welbeck and Rooney potentially playing wide roles (not saying they all will, obviously). I want Januzaj to get a fair bit of gametime next season (30+ games) and so if we are buying a Sanchez then I would think at least 3 of the above players would need to go.