Rashford is still a young player that is talented, he doesn't mean because he didn't perform very well like last 4 years in Manchester United he will perform poor in Barcelona, am very sure Hansi Flick what to used him next season that is why he brought him to the club. Barcelona really need champions league next season because Is the only thing they lack las season so Hansi Flick need to build new players to help him perform very well. And about Rashford and Rafinha and can says both do well last season, Rashford do is best in Aston Villa and perform very good in the club. Let see how Hansi Flick will do is next season lineup, I know if Rashford will always come from substitute or not.
I saw a video of Rashfords Training sections few days ago and I can not agree any less that he is going to do even better at Barcelona, it is a very different environment from what he had at Manchester United and the experience is going to be different to so I will go with the idea of judging him with his last Manchester United performance and think he will be doing same with Barcelona, here he will be willing to show value so that he can earn some recognition and Importance to the team so it is obviously a different mentality all together.
You are sounding like Marcus Rashford didn't play in Manchester united to show importance and to be recognized off course he did as a matter of fact the very first time he went to Manchester united he did absolutely well and that was why his name was known and back then you can not mention two great players without mentioning Marcus Rashford because he was of great value based on his appearance but he just flop lately, I don't really know why perhaps he was tired of the club or..., right now I can not tell for sure if he will do well in Barcelona or not but what I'm sure of is that he use to be a great player in Manchester united before and as such I believe that skills and potential are still in him.