Ultimately, it depends on his bone thickness. Can't say either way, as I can't find a read on his wrist thickness (best indication of bone thickness), but if the bones are big enough and his bone marrow can supply an adequate amount of red blood sells to fuel his muscles then the stamina issue is okay.
Mobility on the other hand; well I suspect the stiffness is already present, so I doubt we'll ever see a fluid-looking Joshua, unless he balances his workouts.
With regards to the fight - I think despite the obvious jump in 'class', I don't see Johnson as an upset kind of guy.
Joshua could well look terrible but I still think Johnson, for all his showmanship outside of the ring, is a missive let-down inside of the ring and will never achieve any kind of genuine success ever again.
He'll find a way to fall short.
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