To be honest, I like the fact that we are playing him out wide (even though it might not be an intentional ploy), and this might even benefit him long term. Playing through the spine of the team carries an immense amount of responsibility, and although Martial is extremely mature for his age, no 19 year old should have to carry the burden for a club of United's stature. Let him play on the left and right wings instead of being the focal point of the attack, let him develop as a wide forward to add to his arsenal of skills as a lead striker for the future, let him make mistakes out wide (the kind that would be more heavily criticized and punished in central positions).
There's no real hurry, he has a lot of growing up to do, and doing it in a position with lower responsibility will take the load off a bit. As the lone striker he'll be the center of the opposition defense's attention, his tactical freedom will be curtailed, and we won't get to explore the extent of his versatility. Even at Monaco, he was essentially playing as a left winger for most of the time, no it's not exactly alien terrain.
Kinda like what Simeone does with his young central midfielders Óliver, Koke and Saúl. Eases the burden on them while Gabi and Tiago are still around, gives them more freedom while also accounting for match experience, but they should be ready when the experienced ones hang up their boots or change clubs. Rooney starting as the #9 isn't the ideal situation, yes, but then again he'll stink it up wherever he plays. Even the likes of Benzema and Suarez played out wide on several occasions when they were Martial's age. And Robert Lewandowski often played as a wide forward when Lucas Barrios was the main man at Dortmund.
