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.
Is Rashford really planning to move to Barcelona? It seems I missed something. Honestly, I think it would be very difficult for him to find a place there. Firstly, the competition is very strong, and secondly, there are young players who clearly surpass him in speed and productivity. So even if Rashford somehow joins Barcelona, he will most likely end up sitting on the bench most of the time. His career in England hasn't been easy either. At one point, he was performing well at Manchester United, but then his form declined, and even Manchester United fans accused him of becoming lazy. Things improved a bit at Aston Villa, but it's no longer the same Rashford we used to know.