Signing Mount means we might see a different system next season because ETH and the scouting team couldn’t find the right player that fits Frenkie de Jong's profile. When ten Hag lost Frenkie de Jong, he changed the way how he set up from 4231 to 433. The following below is when Ajax had the possession and start their build up play vs Dortmund in 2021/2022 season. You can see that Gravenberch is nowhere near in Ajax build up play phase to progress the ball and he used Alvarez as his no 6.
Casemiro role will be similar to how ten Hag used Alvarez. Alvarez is not a deep-playmaker or he is not someone who is known with his passing so he was given role to be less involved in the build up play. You can see below Alvarez (21/22) stats compared to Blind, Timber, and Martinez (The below are 21/22 stats not 22/23 stats). Alvarez is shown with much less passes attempted and less progressive passes in comparison to the others.
I think what ten Hag wants to build a set up that is similar to his 21/22 Ajax system. Our build up and attacking transition will be something like this:
GK
Varane Martinez
Casemiro
RB —————— Shaw
Antony - Mount - Bruno - Rashford
ST
Bruno and Mount will play in Berghuis and Gravenberch role. While Casemiro will play in Alvarez's role. So to answer your question, it makes sense if this is how ten Hag wants to set up.