Promoter Eddie Hearn is considering a "plan B" for Anthony Joshua and says Tyson Fury must clarify this week whether the pair can fight in three months time.
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.
Anthony Joshua Photo: Photosport
Wilder's team had long said they had contractual rights to a third fight with WBC champion Fury.
The September deadline means Fury's team are likely to consider paying Wilder to step aside, but there are no guarantees the former world heavyweight champion will accept that.
"This negotiation has been going on for three or four months and we were always assured this wouldn't be a problem," Hearn said.
"It was very strange decision from the arbitrator to say the least, but that's their business.
"We can't control that or be involved with that process. As far as I understand it, Fury wants to fight Joshua. Hopefully they can resolve the issue and we move forward but over to them.
"We have provided an unbelievable opportunity to Fury. We want to be in a position by the end of the week to know are we fighting Fury or moving forward with another option.
"It's exciting, it's disappointing and it's boxing. Let's see if Team Fury can pull something out of the bag for their fighter. I think he will be disappointed."
On Tuesday, Fury told the Daily Mail he was "in the dark" over the situation but was "fit and ready to fight anybody".
Ukraine's Usyk, who has 18 wins from 18 bouts, holds the mandatory challenger position for Joshua's WBO title.
Joshua was set to risk his WBO, WBA and IBF belts against Fury in boxing's first heavyweight contest for all four recognised world titles.
-BBC