I think Marcus Rashford will be used to rest Raphinha or Yamal in some matches. Barcelona will have a busy schedule once the Champions League starts. Therefore, they will want to constantly rest their key players. I understand their point. Marcus Rashford is currently in the process of adapting to the team. If he gets through this process quickly, he could fit in more easily at Barcelona. He could also be a very good substitute and become a wildcard.
That was the plan in the beginning. Barcelona last season was having difficult time to have a better replacement when Lewandowski went for injury, during that period Yamal and Raphina were the leading players in the front line for 6 games in 6 weeks and they loss about 4 games, they were demoted from the top of the table because of that incident but immediately Lewandowski came back, things came back to normal but not long before Yamal also went for short injury and this affected the team badly.
There are times Rashford might even start some games depend on the situation on ground. If other players that are protecting the front line are not too fit to start, he might be used to start the game more often but wouldn't be allow to finish complete game except if he is doing wonderfully but as it is now, there is nothing big about him yet, he did played 30+ minutes game and there wasn't anything great even though they won the game but who knows, we are just getting started.