I was thinking during the game with Arsenal, he's our most aware player in the final third, he's going to be crucial in whatever attack Rangnick goes with as he's the only forward we have whose first instinct is to create rather than shoot. Still not sure why he played LW and Rashford RW though.
For me you answered the question your self.
Rashford is a forward and is a pure inverted goalscorer from the LW. When he plays as a LW - his first instinct is to use his stronger foot to take a shot and score because of the angle. Its the same as Ronaldo when he was younger on the LW compared to how he played on the RW.
On the RW the angle is closed up for Rashford use his stronger foot to shoot - so he crosses much more than take unnecessary shots.
This is extremely important now because we are playing Ronaldo as a Striker and our main tactic to score- a player who isn't exactly creating chances for inverted forwards either.
So Carrick used Sancho as a LW (most creative winger we have) - then play Rashford as our RW to make him play more creatively as we saw from that assist. Makes two wingers create chances for Ronaldo + Bruno + fullbacks.
If Greenwood was not ill from covid then I'm pretty sure Carrick would have used him as a left winger- let him get used to whipping crosses in to Ronaldo with his left foot.