Joshua's problem will be staying switched on for 12 rounds. He can try and box his way to a decision all he likes, but Ruiz wont have any fear whatsoever. He knows that A, he can hurt and drop Joshua and B, he knows he can take Joshua's best punches. There isn't any psychological barrier there in that regard. Joshua could easily try and box at range for 6-8 rounds but when Ruiz puts it on him, is he going to get dragged into a brawl or try and keep to the plan?
Ruiz has a chin, has enough power and has the speed. He wasn't lucky in victory last time out, he won fair and square. This is a proper 50/50.