Rashford can't afford to be lazy if he wants to become a key player in the Barcelona squad, and he must also be able to work with the existing players in the squad. If he continues to show his ego during any match, he may not get more minutes in the Barcelona team and won't be signed permanently by Barcelona after his loan period ends. I also want to see how Rashford adapts to the other players at Barcelona, as they were generally good players, judging by their performances last season.
Rashford had good talent but his mentality is weak and he seems to lack of enough ambition for success in his career. Barcelona have many star players who performed well in the last season and they can maintain it well in a new season or even do better. Hence, Rashford must know that he will start their on bench as one of substitutions and must prepare as best as possible in training sessions in order to take advantage of any given chances.
If he perform well several times as a substitution, he will increase his important role in the club firstly as one of best substitutions, then moving towards a position in Barcelona main squad. It won't be easy and won't happen after one match or some early weeks in Nou Camp.
At Camp Nou such weak mentality would be changed because Barcelona is not Manchester United that would tolerate some certain things so he'll have to comport himself, work hard like his current teammates and try to impress the coach so he won't be benched too often. Rashford is a star as well but over there at Barcelona he'll earn his spot and it won't be handed to him based on coaches preference but his hard work and effort to do better.