Casemiro is still a very solid DM the thing is that his slow, he needs to be paired with a skilful player that's fast so when he loses the ball, he can track the player while Casemiro will mark the free player that he intends to pass the ball to. Look at the game against Fulham, when Amorim removed Casemiro, that was when the deep in the midfield was totally open because nobody to do the dirty work in that position. For those people that still thinks Manchester United doesn't need defence and midfield, I hope they are watching what's happening to the team in this past two matches.
Let’s be realistic Casemiro is old and cannot do all this work again, let’s not sugar coat that he is slow it is simply is age and nothing more. If you take Casemiro out you will see that the midfield was empty which shows that Manchester United needs to get a player younger with Casemiro qualities at his younger age. For me I don’t think Manchester United need to add more defenders but rather they should add a midfielder most precisely a central midfielder and then a goalkeeper, this two positions are crucial for now but Amorim also needs to learn how to make use of what is at his disposal at the moment