Joshua will have to be aggressive. I reckon he has 5/6 rounds to stop Usyk. If the Ukranian is still upright after AJ's early pressure, Usyk wins by late stoppage.
What doesn't help Joshua is that Usyk is like Canelo and GGG in the fact that he stays within range all the time, these fighters don't even need to throw shots, the opposition seem to burn themselves out having to constantly work to try and keep them away, this doesn't suit Joshua at all because he doesn't have the tank to fight like that. Its a 6 round fight for Joshua this time, if he doesn't knock him out then he at least needs to put some damage on Usyk in those rounds to change the dynamics of the contest.