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.
That's if his performance is good, but if it's not, there's not much to say about Rashford. Because when an average player is in a team that already boasts more excellent players like Barcelona, that player will naturally be compared to the players already in the team. So, for Rashford, hard work is an extraordinary effort if he wants to perform significantly better than the players already at Barcelona. But if the support from his teammates can continue to come for Rashford, I am also sure that he will be better at Barcelona, but if you look at Rashford when he was still used as a main player at Manchester United, I feel doubtful that he will immediately be good at Barcelona.