I personally believe Rashford will do fine at Barcelona and he will rediscover his long gone love for Football and will shine again. However , it's really up to him to make all this happen and also to his new teammates to make him feel comfortable as soon as possible but I'm sure Flick knows well what he is doing so I trust him. Overall , Barcelona looks prompt and ready to win another LaLiga while Real starts a new chapter with Xabi Alonso so I personally don't expect much from Real in his first new season. 
Well, they already have Raphinha on the left side so why did they brought the spare man, Rashford Maybe they will utilize him as the Raphinha sub because Hansi Flick can't substitute Raphinha due to his great performance, and Marcus Rashford doesn't have the same skills as Raphina. So I don't think Rashford will get much of a chance in the starting line up. Some people believe that Rashford's deal is a good move but I think Rashford is a temporary relief from bad times. Barcelona prioritizes its own La Masia players, and that is why I believe that Marcus Rashford will not be able to do anything in Barcelona. No player can show his skills in such a short period of time. He needs proper minutes for performance, but I think Rashford will be utilized as a substitute.