I think his pace and finishing mean he is a 25-30 G+A/season player.
Like with Bruno, the question is if his productivity can make up for the shortcomings. In Rashford's case, I think a weakness at LW can be carried because of the potential damage he can do, and his pace, which the rest of the team lacks. Unlike with Bruno vs big teams, where the weakness is in the middle of the pitch, and that's hard to hide.
He's done a TON of damage from the left in big games- City, Liverpool, and PSG previously, and Liverpool this season. That's the way to use him and judge him.