I don't think that Rashford presence in Barcelona will affect any of their current players, because Flick already know the formation and tactics to use. Players will be rotated which is a norm. In the long run, the best performing players will always be among the first eleven. Rashford should try his best and cope with Barcelona system for easy adaptation and to win a place in Flick's first eleven.
Marcus Rashford's presence will not effect any player. Assuming Barcelona does not have any financial issues, given their present success, I don't think it's necessary for them to bring in more players because every player is still doing their best, and every position is full of challenges. Rashford joined Barcelona at the right time because his position has been Barcelona's weakness, and Raphinha has been managing it well under Hansi Flick. However, benching Raphinha would still be difficult, but only if Hensi flick do some rotation, and I feel that if Marcus Rashford adapts to the squad team, his presence would be a valuable addition to the team, he only needs to work hard to gain a spot with Hensi flick.