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.
Raphinha is a key player who is highly regarded by Barcelona in every match, so he's only substituted when he starts to get injured or starts to tire in the final minutes of a match. Therefore, Rashford may have to look elsewhere, although he shouldn't forget about playing in the same position as Raphinha, as Raphinha won't be substituted casually, especially given his significant contribution to the Barcelona team last season. So, besides having to adapt to Flick's tactics, he'll also need to be more patient when not immediately given full play by the coach.
I believe Marcus Rashford took this into consideration when signing. He has many quality players ahead of him. One of them is Brazilian Raphinha. He put in a fantastic performance last year, helping Barcelona climb the ladder. This year, he'll likely rotate alongside him in his position. If he wants to play more matches during his contract with Barcelona, he must first defeat the Brazilian in the battle for the shirt.