when people like yourself stop thinking Mahrez plays like he did in the title season every game it'll help the discusion, its not coincidence given we are heavily reliant on him, that when he goes into sulky CBA mode we look shit and people moan about us being boring and lacking creativity. Mahrez no doubt has a higher talent ceiling then an awful lot of players, Son included but the base level of performance you can expect of someone like Son is far higher than Riyad often delivers, far too often we are effectively playing with 10 men because Riyad doesn't fancy it.
That said if you are going to run a player to player comparison Lamela would be the player he would actually be competing with.
I'd take Son though not necessarily as a direct replacement for Riyad