The sports scientists felt that Moura could only last 70 minutes and needed to come off if we were winning, drawing or losing. So they did that.
The sports scientists felt that Son could last longer than that, and that Lamela should come off after 80 minutes win, lose or draw.
That leaves just one sub option.
I agree Son is more important than Moura right now, especially with Kane out, but those are the facts, no flawed logic.
Perhaps they felt Son had a 2% chance of injury in the last 20 minutes and Moura had a 50% chance of injury in the last 20 minutes and decided to take Moura off, that is what happened, that is the logic Poch and Co used, Son made it through fine, whereas Tammy Abraham (who wasn't fully match fit) went down with a thigh injury in the last 5 minutes because his coach decided to leave him on.