Britons Fury, 32, and Joshua, 31, had agreed to a heavyweight title bout in Saudi Arabia on 14 August.
But earlier this week a US judge ruled Fury must fight American Deontay Wilder for a third time by 15 September.
Hearn, Joshua's promoter, said the "game changed" with the ruling.
"We have to have a plan B in place. The one that springs to mind is the WBO mandatory - Oleksandr Usyk."
Less than a week ago Hearn told BBC he did not have any back-up plan in place because he was confident Joshua-Fury would be finalised.