Rashford don't need to be reminded, he knows how often he will be benched if he fails to meet up with Flick's standard. His debut game will be a deciding factor, My presumption though , I think Barcelona is full of stars that are capable of benching him if he underperforms. I am certain Rashford knowing this is set to show the world the class of player he is and how good he has become after playing few games in Aston Villa while on loan even helping them qualify for Europa league group stage.
Of course he knows, Barcelona is not where to joke, flick needs them hot for the battle ahead for next season so I believe rashford wouldn't want to be to be use an experiment for the bench, although I expect him do better in that club, yea many talents are in Barcelona that can bench him as you said but that's if he doesn't do it right but I still doubt if Rashford won't do well while in Barcelona, you know that he is meeting with a different team that is competitive, the payer eagerness to do their best will make rashford to try as much as possible not go to the bench, you know sometimes what triggers players to perform very well sometimes is when the teams is competitive because no one wants to be in the second eleven or want to a second half or last minute substitute.