I agree but I think the choice in Emery’s mind is Auba versus Iwobi, not Auba versus Mkhi.
Emery just hasn’t wanted to play a front three of Auba/Laca/Iwobi, partly out of doubts regarding Iwobi on the right but also likely partly because - much like Wenger - he prefers his right winger to be more of a wing midfielder that helps in the buildup and plays in less advanced positions, therefore providing more defensive protection to Bellerin and allowing Bellerin to surge forward into a wing position when in possession.
Its a dilemma because Iwobi is having a great run of form and looks like a potential breakout player. But I think its really a pick two out of Laca/Auba/Iwobi situation when drawing up the starting eleven. Obviously things can change during the match: If we are behind Emery is going to become more adventurous.