Joshua should be able to beat a guy like Ruiz simply by fighting tall and keeping him on the end of his jab. Immediate rematch and do things properly.
He should fight him again as soon as possible. Of course Ruiz is dangerous, but so is everyone else we would consider a worthy opponent for AJ.
First and foremost we are losing some of that mass, 2ndly we are doing a warm fight vs Parker if I had to go directly into the Ruiz 2. We are gonna be light on our feet and jab and any time he gets close we are hugging him. The biggest thing I Joshua gasses he needs to do like Canelo and fight in Spurts throw some punches early in rounds dance in the middle and the last 30-45 of each round he needs to be active so he can steal the rounds and fight off the jab. Flick the jab no matter what throw more jabs than power punches unless he sees an opening.
If Joshua wants to have a long career, he’s probably better off avoiding or delaying the rematch. See what Wlad did with Sanders or Brewster. The risk that Ruiz poses for Joshua is right now is less than Sanders for Wlad and more than Brewster.
Take a long rest - somewhere remote away from distractions. Somewhere like a tropical island and away from isolation. Gather your thoughts and regroup. Come back, and sit down and analyse where things went wrong. Be honest with yourself, admit your weaknesses. Start light sparring and begin working on your weaknesses. Try to re-structure your 'default' fighting style and mindset.