I believe Marcus Rashford took this into consideration when signing. He has many quality players ahead of him. One of them is Brazilian Raphinha. He put in a fantastic performance last year, helping Barcelona climb the ladder. This year, he'll likely rotate alongside him in his position. If he wants to play more matches during his contract with Barcelona, he must first defeat the Brazilian in the battle for the shirt.
Rashford is certainly prepared for this. He is undoubtedly aware that it will be challenging for him to secure playing time at Barcelona. Especially since Raphinha and Yamal are key players who are rarely substituted. However, I believe Rashford does not need to displace anyone in the team. Barcelona will face a packed schedule. This is why Rashford will be used frequently. Hansi Flick will rotate players in some matches deemed easier and give Rashford an opportunity.
Remember, Barcelona will definitely be targeting the UCL this season. And for that, the team needs good substitutes. So they can still perform well in all competitions, whether in La Liga or the UCL. With Rashford in the second tier, I think Hansi Flick will truly have no concerns about the busy schedule in the upcoming season.