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.
Rashford and Raphinha both have their strengths, but it's true they can’t both start in the same role right away. Rashford might need to come off the bench or play less critical matches at first to adapt and build rhythm. Over time, if he fits well in Flick’s system, he can gradually earn more minutes. So, it’s not about replacing Raphinha immediately, but creating balance and options in attack.
Flick has his reasons for going for Rashford when Raphinha was one of his best players for last season now both players can not start a the same time but who knows if Raphinha is not feet like last season that could be the reason why Rashford is in the picture already it could also be that they need a younger player that could replace him at Barcelona so that it will not be difficult for them when his contract expires with the team and they are not willing to offer him another contract because of his age. Raphinha should be a regular starter if his feet but the coach will have the final say as for who getsore playing time among the players.