I mean hindsight is 20/20 but up until the 3rd, there was no reason to believe he would KO'd to be honest. Ruiz' face was starting to get bruised from the jabbing and AJ connected some very good shots too. As a matter of fact, after Ruiz' KD, AJ landed a huge straight and some hooks which Ruiz managed to eat well. I think man was just to eager to finish and end up getting caught badly which totally derailed him. He may not be able to improvise indeed (I don't dispute that) but I think his loss today had more to do with him being badly rocked than him not being able to change gameplan on the fly.
I think he always had an average chin and except against Povetkin where he ate some shots but still kept his legs, Whyte, Wlad and Ruiz managed to give him noodle legs the moment they connected clean. Only difference IMO is Ruiz' handspeed and relentlessness against which he couldn't keep up. When Whyte rocked him, Whyte tried to land some huge hooks AJ could see coming from a mile away. When Wlad rocked him, Wlad tried to land his lead hook repetitively with almost no success which AJ also saw coming. Both wanted to land in single punches to end AJ but Ruiz went in with fast combos forcing AJ to shell up, completely nullifying his offense and giving him no time to rest and regroup.