Raphinha is in his best form these days. Why would Hansi Flick decide to cut him from the starting lineup to "rest"?
It doesn't make sense while he is one of top goal contributors. Rashford will need to earn his place there. He will need to work hard to stay fit and impress the manager. I can't say he has been in his best physical shape recently.
Rashford could play both wings and forward. Which means, when Raphinha needs rest (these players play 50+ games a season) then Rashford can play, plus he can join later in the game, like 70th minute or 80th minute to bring in some fresh legs. He could not only help with Raphinha, but also Yamal and Lewandowski all at the same time.
It means, we are going to see Rashford definitely have plenty of time to play, he is going to play many games, both subbed in, and even starters to rest others. He is not going to replace as in be the starter or preferred player, he won't be, Barcelona has better players, but he will certainly be the first sub to look for, could be one of the best if not the best bench player they have in any given game.