Lets not forget SMS is more then capable of playing in an advanced position either being a number 10 in the 4-2-3-1 or even as a left winger, If played on the left wing he likes to cut in and also drop into the midfield, its a nice option to have especially against teams who always try to out number you in midfield eg. Man City, having him drop in to the middle would help as win more midfield battles. He'd also be more of an aerial threat in an advanced role, If he played on the left we'd need to provide decent enough crosses from the right and he'd always be a threat at the far post. Obviously he wouldn't be as direct as Martial and Rashford as a left winger so it all depends on the strategy we would implement from game to game nonetheless he would be a great asset as he's a versatile player.
So all in all he can play as CM, CAM, LAM/LW.
In my opinion he best suited to playing as a free roaming CM, mainly looking to attack and create but at the same time helping in defence by intercepting, tackling and just using his shear physical presence. Iv'e seen him play in the other positions i mentioned and he was more than capable, so if we sign him there's a number of positions he could potentially cover if players are injured etc.