I actually think Rashford is better off on the left, than in the center. His strength is running with the ball into space, and his finishing is decent. I have my doubts about his hold-up play, ability to trap the ball from the air, springing the offside trap, or even picking the right pass. And I do not see him developing those areas to function as an effective center forward.
For me, he'll be someone who is best used to gather the ball near the half-line, dribble forward, and cut inside to shoot / pass central. Wing is his best place where interplay instructions can be simple, and we can utilize his strengths.
We have been terrible at developing players ... players in the last few years have come here and regressed. That being said, I don't think that's the case with Rashford. IMO he's been rightly pegged by multiple coaches as a wide player / wide-forward.