Martial wasn't playing centrally; he was alternating along the front line with Rooney and Lingard. All 3 were working their socks off; Martial did a lot of his work on the wings. Aside from the first Goal Martial was ineffective in the centre, he lost most of his aerial duels and was pushed off the ball quite often, Burnley physically dominated everyone on the pitch except Fellaini, Pogba and Bailly. Herrera was good this game but he was pushed off the ball easily a few times.
I don't think Martial should ever play in a 2 striker system. He doesn't have the hold-up play or off the ball movement to play in that system. Rashford is much better suited to that. I think the best situation for us is to get Griezmann and play him, Martial and Rashford as a fluent front 3 (also including Mata and Micky as well). A 2 striker system is something most managers avoid because it is too static, they either choose 1 striker and accomodate his weakness into the team (Ibra, Costa, Lewandoski etc.) or 3 fluent attackers.