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.
It will not make any sense to put Raphina on one side and start Rashford and at the same time, if Rashford isn't used properly, they can't know his potential potential and also considering the fact that he is just having a year contract with the club, they just have to use him unless they have money to waste on a product they don't want to use. I am just looking at what Raphina brings do Barcelona in the Champions League last season, too good to be ignored.
Perhaps the team will want to give each of them fair share of play, it's left for the players to make use of the time they are given. If Raphina is given first half of the game, it's left for him to deliver the number of goals expected and same goes for Rashford and I think this pattern of share time will not only make the players serious, it will make each of them want to deliver good performance because if they don't score on their time, they have to wait for another match to deliver again.