Scholes played as a forward, Neville was everywhere and so was O'Shea. Macheda came on as a striker but later when he was given more starts it was out on the wing because he wasn't good enough to establish himself as first choice striker yet. So what did SAF do? Did he just stop playing them? No, he played them in different positions. Every manager does it. It's even more prevalent today because tactics aren't as rigid. United used to just play 4-4-2 no matter what but now you need to be more versatile.
Soery but your memory of this is wrong.
Scholes was a forward when he first broke through. That where he'd always played in the youth teams. Kind of proves my point. We didn't just switch him into midfield on his debut.
P Neville was always a fullback when he was younger. The switch into midfield came years later. Oshea was used all over but then that's how he remained throughout his career. His position was as a utility player.
Giggs, Sharpe and Beckham were introduced as wingers. G Neville as a fullback, Brown as a centre back. Butt in centre mid.Their actual positions.
If there's an actual reason to play them out of postion then maybe. Fletcher was used out wide because the alternative was an inconsistent Ronaldo who was a defensive liability. There was some sense to it. It didn't do anything for him personally but for the team it helped.
There is no firm, tangible reason for Rashford to be on the left when we have other options as good and actually better than him there.