Promoter Eddie Hearn is considering a “plan B” for Anthony Joshua and says Tyson Fury must clarify this week whether the pair can fight this summer.
Britons Fury, 32, and Joshua, 31, had agreed to a heavyweight title bout in Saudi Arabia on 14 August.
But on Monday it was ruled Fury must face American Deontay Wilder for a third time by 15 September.
Hearn, Joshua’s promoter, said the “game changed” with the ruling, adding: “We have to have a plan B in place”.
He added: “The one that springs to mind is the WBO mandatory – Oleksandr Usyk.”
Less than a week ago Hearn told BBC boxing correspondent Mike Costello he did not have any back-up plan in place because he was confident Joshua-Fury would be finalised.
On Sunday Fury announced he had agreed to the bout.
Wilder’s team had long said they had contractual rights to a third fight with WBC champion Fury, and a US judge has ruled in arbitration in their favour.
The 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.”
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.
Analysis – what will Wilder do?
BBC Sport’s Luke Reddy
Hearn has been full of positivity and conviction throughout, so for him to now say alternative options are worth considering is particularly noteworthy.
Fury’s promoter in the US, Bob Arum, apparently has Allegiant Stadium in Las Vegas booked for 24 July in the event a venue is needed for a Wilder fight.
Will Wilder step aside and take a healthy pay-off? Saudi authorities are putting up more than £100m to stage Joshua-Fury, so there is ample money flowing through the fight to hand out.
But why would Wilder’s team go to this amount of trouble to then step aside? And will they risk placing control of the heavyweight division in the hands of others when they have fought to take a piece of control back themselves?
In 24 hours we have moved from Joshua-Fury being all but nailed down to observing a heavyweight mess.