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Raphinha as midfield? I don't think so, if Hansi Flick needed a midfielder he would've signed one instead of converting one of his best player that thrived in the left flank last season to the midfield and displace two other important players, makes no sense. If you have said he rotate between Raphinha and Rashford on the left flank then it's understandable but to play Raphinha out of position just to fix Rashford that was bought on loan is not a good idea, it seems you forgot that Raphinha is a very important player for Barcelona and his best position in that team is from the left flank, Flick won't take that risk of playing him in the Midfield.
Hansi Flick has a plan to utilize the new formation where he will use 4-2-3-1. Four men will be in defense (Kounda, Cubarsi, Martinez, and Balde) 2 men will be used as holding players ( Pedri and De Jong). The 3 player will be used as attacking midfielders, where Yamal Jamal and Marcus Rashford will play widely as wingers, Raphinha will be used in a CAM position and the front leader will be Robert Lewandowski.
So that's why many people believe that Marcus Rashford will play full time like he is not a backup of Raphinha, and honestly, if we look at his formation, then apparently it makes a sense because there will be four attacking players rather than 3. So we don't need to take intense concern because Hansi Flick is a great coach, and he knew what the good for the team.