Manchester United's forwards, midfield and defense are all very weak. So I see no reason to think that Casemiro in the team will actually strengthen every position of Manchester United. Casemiro is an experienced player. But in the 2021–22 La Liga season, he scored just one goal and 3 assists in 32 appearances. So I don't think United will be strengthened by Casemiro.
Rating Casemiro performance based on the number of goals and assists might not be accurate. A defensive midfielder should be assessed based on the number of successful tackles, ball won, distance covered and number of blocks. The player is fast and physically strong, hence he is a very good asset to the Manchester United team. Real Madrid is selling him because the really need the money and because they already have a replacement for him (Asier Illarramendi). Manchester United are just waking up from their long sleep.