The fact that Rashford ended up at Barcelona is a big stroke of luck for him, and how he will be able to take advantage of it will depend on him. There is a lot of competition at Barcelona, and the fact that Flick is giving him playing time shows that he trusts him, or at least wants to see what he is capable of. I think that out of all the possible options, this was the best one for Rashford, and if he somehow manages to become a first-team player, I will be happy for him.
I am pretty sure that we are going to see Rashford not be that amazing here eventually too. The problem he had at United wasn't really just about how terrible United is, because he was having trouble at United even before all of this United becoming terrible period. He is "known" to be lazy, and if he is lazy then he will not be playing here too.
He is getting some time there, and that is great so far but give him some time, he is new there so he wants to prove he can play at top level still, but when the league ends then we can look back and see that at some point he got bored again and stopped working out again. That's not a guess, that is historical pattern and I am pretty sure that he will do the same thing here too.