Rashford simply needs to play according to the coach's instructions when he's deployed in his preferred position, as the Barcelona coach will then assess Rashford's level of cooperation with his teammates. Barcelona is currently striving to make significant improvements in order to have a better season and free itself from financial difficulties, so Barcelona should be able to win several major competitions next season. And Rashford's arrival at Barcelona could at least add a little strength that could have a positive effect on the team as a whole.
In a match, every player must follow the coach's instructions because they have carefully prepared their strategy. However, Rashford's problem goes beyond that; he needs to train harder, especially improving his individual skills and working well with his teammates. Rashford's arrival at Barcelona provides more space, as the coach has the opportunity to rotate players if desired, given the busy match schedule next season. Rashford's abilities are also excellent when he's in top form, which is an advantage for Barcelona.
Rashford joining Barcelona is going to affect some players in the squad, players like Dani Olmo and Fermin Lopez, this players play as a midfield and i believe Hansi flick will use Raphina in the midfield in some matches and Rashford to play as a winger, Rashford might not complain about the minutes he play but he will not like it when he is being benched more than 5 matches.
Dani Olmo and Fermin Lopez are star players and they will want to contribute to the team success and without not having much time to play they will feel like they are not important to the team, Rashford will be very important to Barcelona most especially when a player like Raphina or any attacking player gets injured but there is still going to be a problem, Rashford will also stop the way he play in Manchester United if not Barcelona will not have any intention of buying him permanently.
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.