He’s, at best, a sharp penalty box player with fantastic kicking technique. Beyond that, his dribbling is awful and his football intelligence is substandard. He’s only going to be a Vardy/Hernandez-type finisher, and that will only suffice for a top club from the bench.
Mason Greenwood will surely eclipse him within 18 months - his natural talent and football IQ already seems to be superior, albeit at a less significant level but you can tell he has an elite aura around him that Rashford lacks.
For what Mourinho requires from him at the minute, Welbeck would be a better fit - for all of his limitations and chaos, at least Welbeck actually used to provide a Heskey-esque outlet, be able to hold the ball up and occupy the opposition to benefit others rather than just kick it down the line and chase.