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.
More than anything, Flick must also look for a worthy replacement who can replace him in the worst moments or in the moments in which he does not feel 100% and it can happen given that he is a player who always plays at the top of his game, so it is better if he finds a replacement as soon as possible.