For someone who lies for a living, he was really struggling with the idea of just flat out lying right now. He feels a pang of guilt even as Adrien is the one to lie because he goes along with it. What's more is that it grows as Joss asks for details. He tries his best to tell himself that if this is what Adrien thought to be right, then he shouldn't challenge him on it. Not a petty white lie like this.

But he's going to let Adrien cover for the soreness and the pain, "You were completely passed out."