@Alphabets -- on a general scale, you're correct that a player doesn't have to touch the ball to impede play. However, there are limits to where that applies. Basically, you could call Rashard for active involvement while running after the ball IF he also prevents a defender from playing the ball. That does not appear to happen in the video. The other way it applies is if the offensive player in an offside position shields the GK when another offensive player shoots on goal. That's not the case in the video, so it wouldn't apply in this situation.
So while the play looks unusual, because Rashford didn't touch the pass (as best as I can tell; there is one point early in the play where it's tough to tell if he got a small touch or not) while Fernandes, who was onside, took the shot and scored, it was called correctly. The ref instructions for a pass to a player in an offside position is to keep the flag down until the ref sees who actually plays the ball, in case either a defensive player or another offensive player who is onside is the one that actually plays the ball first, which is exactly what happened on this play.
