The 22-year-old was called up to the Netherlands squad in June.
The Crystal Palace defender, on the receiving end of a Darwin Nunez headbutt, received abusive messages after the 1-1 draw at Liverpool.
Promoter Hearn believes Joshua had a heavy burden lifted from his shoulders following his defeat to Ukrainian Usyk in London last September.
The Proteas skipper also insisted nothing should be read into their heavy defeat to England Lions last week.