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.
I don't believe that Rashford is signing Barcelona only to be a substitute player. Maybe he will become the new left winger, while Yamal is going to stay as a right winger. Lewandowski is getting too old and Barcelona needs a new central striker. Barca signing Rashford seems like a risky transfer for me. Some English players become really successful playing in Europe, while others get injured or their performance deteriorates. Rashford never had a good performance in the last 2 years in Manchester United. I'm a bit skeptical that he will suddenly become a great player in Barcelona.