Mou seems not to expect pressing from Moura. Mou gave an attaching role to Moura more than Son. Maybe Mou thinks Moura is better than Son as an attacker. So, we need to sell Son. One of Son and Moura should do pressing and Son doesnt have good pressing skill.
I'm pretty sure he was pissed at Moura for constantly failing to track and press his man as Olympiakos constantly got overloads down our right, which is why he hooked Moura and not Son.
Son is not good without the ball, but he's a better all round package than Moura, who has no brain.