Strange to find people frustrated as his today performance. Here I am talking about his game when he was moved to act as left winger/ wing forward.
Mourinho's instruction for him probably is to stretch and draw out Simpson from Leicester's usual compact narrow back 4 defending line; then pass back to a narrow supporter (Even Rojo & Blind make run in side than overlapping), who often play a pass in between Leicester full back and CB. This space is previous made by the run of Rashford. Mata's goal on the other wing is similar play.
To understand it better, let's talk about how Leicester plays. They play a narrow back 4 and use the other players to press, forcing the other teams into wing play, which they corner and isolate the opponent wide players while having plenty of bodies to defend crossing (they're shite at defending crossing aimed at their full back though. Problem not many teams have good crossers nowadays to capitalize on this, so Leicester got away with this last season).
As I said we tried to play direct to out "pitch stretcher", here Rashford with his pace off ball or with ball movement, meant Leicester didn't have enough time to reorganize their system. This force Simpson, and their other full backs to move out to stop our "pitch stretcher". Our aim is to attack the space left between CB and FB, which we got it quite often.
Young when came on did similar. He stretch the play then turn back and side pass, then couple passes later, there would be a pass in area between Leicester full backs and CBs.
It's a tough game for young player who often like to play positive and force thing than trying to tactically outsmart the opponent. Of course, there is chance Rashford can cut inside himself and be selfish. However, he overall he fulfilled his role well. Let's not forget he had a chance or two, which he couldn't convert, which some experienced players would grab to mask their poor game. Here is a good performance from Rashford. With experience & familiarity to the role, he may improve and take the few chance he's afforded.