If he doesn't stick behind the jab, I can see Ruiz coming over the top and banging his temple again, they'll have worked on that in training after the effect in the first fight. Joshua's no Lennox Lewis , but he's got an amateur record that shouldn't be disregarded, and he showed against Parker he's capable of going 12 and winning with a comfortable points margin.
Having said that, Ruiz is no Joseph Parker either, he's busier, and there's that hand speed, which really comes into play once he's inside. When the first fight exploded, Ruiz didn't work his way into that space, Joshua stepped into it trying to finish Ruiz off, and ended up getting separated from his senses. If AJ doesn't get on his toes, and stands in front of Ruiz, he'll get tagged, and so will Ruiz, but the Ruiz camp know what hurts Joshua, which is not entirely reciprocal because Joshua pretty much hit Ruiz with the kitchen sink in the first fight, and he kept firing back.They know that like all smaller guys he's vulnerable to that uppercut that put him down, but you've got to let him inside to use it.
I think Joshua could be in trouble. I don't think his corner knew what to do about Ruiz in the first fight, and AJ certainly didn't. They've taken steps to correct that, brought in sparring partners who apparently know Ruiz style better than anyone to try and better prepare him, in contrast, Ruiz just needs to go online to study every minute of Joshuas entire pro career. He's prepared to walk through punches and he does something Joshuas other opponents have rarely done. When Joshua loads up, he fires back. Ruiz didn't, and isn't going to back off him and cover up the way other fighters have.
Joshua is saying he's not there to put on a show, he's there to win, which could be taken a few ways. Going for a lackluster points win, or a not worth sitting down destruction are both possibilities. He may want to send a message just as much as he wants the belts back. He may need to send a message.