The basis for this Darmian quote is the season Rashford was having prior to the injury. But the stats tell you one thing, and actually watching the two player tells you another. Could Rashford be able to put up the numbers Mbappe does? Maybe, someday but even this season I don't think he's there, or the next. But I think he can get in that area of what Mbappe is doing now. But Mbappe is improving at a far quicker rate than Rashford, and the Mbappe of 2-3 years ago is better than the Rashford of today still. The thing with Rashford is that he is extremely dedicated to his craft, he finally has a coach who fully believes in him, and he is now the center focus of the team, and in Martial he has a forward that will put him in good situations, hence the season he had (especially when Martial returned). He's finally in good conditions to improve and this is where his career becomes exciting to follow.
Mbappe really has yet to have a team built around him, he almost never takes penalties or free-kicks, and for a solid half of his career at PSG he has been second to third fiddle at times, so whenever he does move to a tougher league, it will be by a select very few clubs who could afford him, and the team will most likely be built around him. That club will probably be one of the best clubs in their league, so I don't think his transition will be very difficult. It's easy to use examples of X or Y player moving to a tougher league and try to draw anything out of it, but I think special players like Mbappe (or Neymar) will always be on dominant teams and wreck wherever they go.
I fully expect Mbappe, barring injury, to have CR7/Messi like seasons stat-wise once he moves to a team and is the focal point of the offense.