That is not how it works because when Rashford plays, it implies that one of the other attackers won't play and this is where I think his move to Barcelona could end in deep frustration for him. He can hope for injuries maybe, but apart from that I can't imagine who of those other attackers he should adequately replace in the toughest games. I think he will end up as one more option for Barcelona to rotate when they can afford it in games that are not overly important.
I don't think Rashford coming to Barcelona will cause any challenges for him, even though he may not get enough playing time since Barcelona had a fantastic season last season. Rashford will play for the team, which is why he was loaned from Manchester United. Last season, Barcelona had their current players and chose not to sign more players because they were not counting on Ansu Fati, who plays as a left winger like Rashford, but Ferran Torres was injured last season, which could be the reason they signed Rashford to strengthen their games.
Furthermore, I've seen some comments saying that Rashford could bench Raphinah next season, in my opinion, Rashford could bench Raphinah in some matches. But, if I'm not mistaken, Raphinah is not a left winger, he normally plays as a midfielder, but Barcelona used him as a left winger because they didn't have the players they needed last season and still performed well.