I don't remember him missing golden opportunities though. That was Rashford. He had a few ranged shots well saved or going narrowly wide, but nothing you'd call "a sitter" springs to mind. When he plays on the left his instructions seem to be to provide mostly width. He hasn't been in many good goal scoring positions this season.
Regardless, your first comment is wrong in my experience. When I played football in a team, working on your finishing from certain angles was pretty standard for a striker. Like, cut inside from the left, side-step a cone, try to curl it in the far corner. Or run towards the keeper and try to chip him, that sort of stuff. Repetition makes you that much more accurate and confident that you can pull it off during games. If that kind of coaching does not happen, then yes finishing can indeed be affected. It's not even an argument imo.