I don't think Rashford needs to play against average clubs before he should prove his performance at Barcelona. He can quickly get used to the team if he gets playing time with Hensi flick. Rashford and Raphinha almost play the similar pattern. Raphinha is playing the speed game, and considering their current performance, Marcus Rashford has more speed than Raphinha. I'm not trying to rate Rashford higher than Raphinha because there are several aspects that make Raphinha more valuable than Rashford. Raphinha can play a variety of positions, it would be perfect if Hensi flick used him in the attacking midfield and Rashford on the left wing since that is his natural position.
How then will he get the playing time if another player is not benched? Of course he and Raphinha plays the same wing but not same pattern of football. As both of them plays the same wing are you expecting Fick to bench Raphinha in place of Rashford in an important game? Of course not, so Rashford definitely has to play the less important games, come up as substitute as well in other games to gain confidence and balance in the team.
Seeing the current conditions, they will inevitably share a place but of course this is also not a problem especially in the current schedule which is quite a lot so that Flick can now have the option of several schemes that he can do.
As you said, for now Rashford and Raphinha even though they have the same position but their style of play is slightly different which makes this an advantage. Both have similar speed but the difference is in terms of positioning where sometimes for Raphinha he is more likely to give space to other players as a distraction but for Rashford he is more inclined to his dribbling. The only thing I want to see now is that after he passes his medical at Barcelona whether or not there will be any more drama for the player's registration because either way this could be a difficult situation if every season continues like that.