There's no way he should be deployed on the left flank. The best teams in the CL often play nice crosses into the box that find the head of central attackers resulting in goals. These crosses are whipped in before the defence is organized to deal with them. Rashford doesn't appear confident enough on his left foot to play a pinpoint accurate cross from that side. I'm sure if he was playing RF, his right foot could find the head of Zlatan, Pogba et al.
Often Rashford breaks into a deep inside left position and the natural thing would be to shoot with the left foot. The only way to strike on goal with the right in that situation is either with the outside of the boot, or by stepping across, shifting body weight, and then striking across the line of the ball.
In other words the only way Rashford can be really dangerous playing on the left is by cutting inside onto his right foot, but it's not a direct or fluent way of going about it.
I bet if he could break down the inside right channel more frequently the opportunity to score goals would become more frequent.
We really don't have forwards that play dangerous crosses and Rashford on the right would go some way to rectifying this. It is said that Valencia provides width and crosses from the right, but he's the fullback and by the time he gets in position to make a cross the defence is usually well organised in numbers.
Just look at the way Erikssen broke into the inside right channel against Gent and finished with his right foot: it was the natural thing to do. With Martial on the left, Rashford on the right and Mata as #10, that's how we should deploy at Wembley.