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.
I think Rashford's arrival gives Flick a lot of options, which is advantageous. However, I've heard that Rashford's arrival could be a problem because Barcelona's financial situation is still quite dire at the moment. Other news is that Dani Ceballos and Eduardo Camavinga will not be sold, according to coach Xabi Alonso's request, in order to maintain depth in midfield. Brahim Díaz has been retained as a potential replacement for Jude Bellingham, with his contract still secure despite not yet being renewed. It seems Xabi is still looking to strengthen the midfield further, as there are reports he is interested in Manchester City midfielder Rodri. However, his injury history and fitness remain a concern, as he was injured last season and spent a lot of time on the sidelines.
Signing Rashford is the best thing Flick has done to Barcelona this season they missed signing Nico Williams so getting Rashford even though its on loan is an opportunity to strengthen that wing since I heard he wants to use Raphinha as an attacking Midfielder. Rashford would add lots of strength to their depth and he's already fine in the training ground.