Maybe this is me sitting on the fence a bit, refusing to make a prediction so it doesn't look daft. But I am honestly unsure how this goes.
There's no doubt that he could thrive (temporarily in a season) in Flick's system. Score and assist a lot and improve his all-round game. That's best case scenario. Worst case is he's abysmal and kicks up a fuss in the dressing room, or starts getting too big for his boots, starts big games later in the season, and flops in them.
Knowing what an unpredictable individual rashford is, neither would surprise me.