The problem is that most players look better in the center. It's easier to attack from the middle unless you have really top-end pace. How many wide players are really consistently good enough and productive enough to really be nailed on starters at a club trying to win the treble? The list of 9's or AM's good enough is significantly higher.
I think Januzaj will be mainly right-sided player here in a few years once he's productive enough. We just have too many central attackers (Rooney, Falcao, RVP, Mata, Wilson, Hernandez, Di Maria, Herrera) compared to wide options (Di Maria, Nani, Januzaj, Zaha) and nobody else merits inclusion as a long-term option currently on the team or on loan, with all due respect to Valencia.
Rooney will die here and no manager seems to see him as a serious option at LW even though if he could find a way to score some goals from there cutting in our front 6 becomes a lot more logical. That's already one central spot taken up, and we seem likely to buy Falcao and RVP will probably have a purple patch at some point as he's still incredibly skillful if clearly aging.