~snip~
Moreover, I think Manchester United should play Cristiano Ronaldo and Casemiro on a regular basis in the starting 11. As a result of this, they will be able to provide a lot more stability than if they played Harry Maguire. I know that they are not the players in the same position. But I hope you get my point

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Casemiro is surely needed in the starting eleven for Manchester United as soon as possible. Erik ten Hag's main goal must be this in the end. Eriksen would continue to play in a more offensive position and we would see Casemiro playing right beside McTominay most probably. Erik ten Hag preferred him to replace Rashford in the last game to play a more defensive game in the remaining minutes. But I'm pretty sure that he would be a part of the starting eleven after some time.
When it comes to Ronaldo, I don't think that Erik ten Hag should prefer him in the place of Rashford now. Because Rashford has been doing an amazing job since he was started to be given chances. He was the player of the last game too with 2 goals and 1 assist. There is no reason for him to be replaced with Ronaldo for now. The only way might be a two-striker system but it might not be very good for the team while this tactic is working.