If Yaya speaks up, the problem will be solved. If he doesn't, he is effectively backing his agent over the club. Obviously he can have a few days to talk/argue with Slug but he has two options: get Slug to apologise, or if he won't, then Yaya should make it clear Slug isn't speaking on Yaya's behalf. It's easy enough. But if he doesn't then he either agrees or hasn't got the balls to stand up to him. Even if he said 'it's not fair, Seluk doesn't speak for me, why should I suffer?' that hits the ball back to Pep & he has to justify it. If he says nothing then he's got no excuse.
Either way, Pep has put the onus on him to make it clear where he stands, so it's in the open now.