For Marcus Rashford, this will be a career development. Since joining Manchester United, especially in the last few seasons, he has not seemed to improve. His move to Barcelona will certainly mark a new chapter in his career, offering many possibilities.
On the other hand, for Barcelona, this is an advantage, especially since last season, their forward line was often in the spotlight when Hansi Flick didn't have many options for player rotation. Next season, Marcus Rashford will be there for that. I'm confident Marcus Rashford will be able to show his best performance with Barcelona, although this will be a bit difficult due to the competition within the team for playing time.
Rashford was a Manchester United "legend", for a long time, he did alright and he constantly played. Only in the last 3 seasons or so he has been seeing a decline and not only Amorim who is sending him away now, didn't want to play him, but also previous manager Erik Ten Hag didn't want him neither.
So when you think about it, we are seeing two different manages disliking him, and there must be some reason. You don't see that kind of reaction that quickly and Barcelona needs to be careful. Sure he can be a good help, but he could be a trouble too, they need to check what seems to be wrong and what is getting in trouble so that they would know what they are dealing with, otherwise it is not going to be an easy decision for them at all.