Rashford and Raphinha both have their strengths, but it's true they can’t both start in the same role right away. Rashford might need to come off the bench or play less critical matches at first to adapt and build rhythm. Over time, if he fits well in Flick’s system, he can gradually earn more minutes. So, it’s not about replacing Raphinha immediately, but creating balance and options in attack.
Flick will obviously not start Rashford in very vital matches already, he will need to sit back and watch so he can get along and there is Raphinha like you rightly mentioned who is also very good and plays in same position so he will have to wait probably to fit in on a rotational tactics, Flick will need to observe him for a while because he got signed just recently which is not giving him enough time for training before the season starts by next month so in his first season or half of the first season will be mostly to watch the the team pla y and learn to fit in.