Casemiro is a very important player in maintaining the balance of the midfield. Manchester United have been very fortunate to have brought him to Old Trafford this season. He was the first player to win over the ball and would not let the opponent get deeper into the Manchester United defense area. Rashford became a goal-hungry player, he could almost score a goal in every match that allowed Manchester United to achieve positive results.
We can imagine if Manchester United didn't have Casemiro this season, they would definitely find it difficult to maintain their victory in various competitions including the European league. Casemiro's role at Manchester United will never be replaced, even if someone fills Casemiro's position when he is absent there may be another name like McTominay or Fred, but these two players are still not good enough to fill the defensive midfielder position. Marcus Rashford is increasingly improving his performance and it is almost impossible to stop his movements in the opponent's defense area. Manchester United have had a great season so far and I hope in the Europa League they will be even sharper for their opponents.
But they got him, that's what mattered in the end. If Barcelona didn't had Messi, or if Madrid didn't had Ronaldo, they wouldn't be that good neither, but they had them. So all in all, it would be wise to think that what ifs in football do not really matter and what matters is what happened in reality. United managed to build a team that is amazing, and filled that spot left by Pogba with a much better player.
I have absolutely no idea why Real believed that they didn't need Casemiro, sure they have some good young midfielders, but while they were getting older and better, they could have kept Casemiro. In the end that decision helped United a lot and got them so much better as well.