I don't think we should necessarily force Partey, Rice, Havertz and Ødegaard into the midfield at the same time.
However, I definitely think the quality of losing Partey would drop a lot nonetheless simply from the fact that the chance of not being able to field three of them at the same time would increase.
Fielding 3 out of the 4 we have should be possible to do pretty much all the time. 3 out of 3, not so much.