An early KO, how? Against a faster-handed; heavier (by far); more durable opponent, who, as I already said, showed he can take Joshua's best punches at a heavier (more powerful) weight, when he is fresh?
There's no evidence at all that even heavier Joshua could knock this guy out, even if he planted and went all out for it, to then try and do so when lighter - and Ruiz even heavier - coming off a loss, is basically career suicide.
I don't think anyone in the division will be dumb enough to stand and trade with Ruiz unless they have to.
Stylistically, he is the worst match-up for Wilder after Fury.