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.
Rashford's presence is to assist Hansi Flick in rotating the squad, so Barcelona's initial goal was not without reason in borrowing Rashford from Manchester United. Hansi Flick believes Rashford can perform at his best. At Manchester United, he experienced a decline because he didn't play much, so his abilities naturally decreased, but if he plays more often, I'm sure his performance will gradually return. Hansi Flick will definitely give the player a chance, so Rashford must prove himself.