I don't think Rashford would mind being benched.
Nico is a player who plays directly at Bilbao and wouldn't want to be benched at Barcelona. That's why he didn't transfer to Barcelona.
Rashford is a player who plays every aspect of the attack. He'll likely be Olmo's backup.
If he fits in, he'll be a starter right away when he gets the jersey. If he wasn't confident in himself, he'd go to the Saudi Arabian League instead of Barcelona.
I don't think Marcus Rashford would mind rotating with Dani Olmo. Barcelona is a huge team, and his willingness to embark on a new challenge in Spain after England shows us how competitive the player is. We all know he wants to fight for the right to play. I'm sure playing for Barcelona won't be easy, so he'll keep fighting. I think it's a good signing.