Anthony Joshua is already "looking elsewhere" for his next opponent after Tyson Fury was ruled by a court to fight Deontay Wilder again, says his promoter Eddie Hearn.
Fury has been given a deadline of the end of this week by Hearn to resolve his obligations to Wilder and confirm a fight with Joshua.
Joshua may instead face Usyk and then reschedule a fight with Fury to December.
WBC champion Fury had confirmed on Sunday night that he would face IBF, WBA and WBO titleholder Joshua on August 14 in Saudi Arabia in the most anticipated fight in the sport, but 24 hours later a court of arbitration ruled that he must meet Wilder again by September 15.
Asked if he was now looking at a different opponent for Joshua, Hearn told Matchroom: "Absolutely. I had been focusing on Plan A. The only fight was Fury and we hope it can still take place.
"The game changed - we have to have a Plan B in place, and probably a Plan C.
"We have different options. The one that springs to mind is Usyk.
"If Team Fury don't get their act together by the end of this week then we will have no option but to look for an alternative fight.
"If the right deal can be done to maintain the belts and keep the Fury fight alive, but for December."
The undefeated Ukrainian, a former undisputed cruiserweight champion, is mandatory challenger to the WBO belt held by Joshua.
The WBO had agreed to forego Usyk's right to a world title challenge in favour of Joshua vs Fury.
But Usyk's promoter Alexander Krassyuk exclusively told Sky Sports on Tuesday: "There is no other reasonable excuse for AJ now.
"If he is not avoiding Usyk, it is high time to face the mandatory!
"Two London Olympic champions to face each other, nine years later. What a story.
"We are now in the position to make the fight happen as soon as possible."
Can Joshua vs Fury be salvaged?
"Bob Arum called me," Hearn said about Fury's US-based promoter after news broke on Monday night that a court ruled in favour of a third fight with Wilder. "All Hell breaks loose.
"I wanted to speak to them to see what was real, what wasn't real, what could be done.
"It was a shock to the system."
Wilder wanted a third fight with Fury after losing his WBC belt in their second clash. A trilogy match was considered for last summer then December 2020 but a date was never finalised.
"The negotiation has been going on for three or four months and we were always assured that it would not be a problem," Hearn said.
"It was a strange decision by the arbitrator, to say the least.
"But that is [Fury's team's] business, their responsibility. We have to see now where they go with it.
"Arum was in complete and utter shock. I have never heard him so speechless. He has been bullish throughout this process.
"'This wouldn't be a problem, this wouldn't stand in the way of a Joshua vs Fury fight'.
"That is frustrating because we have worked tirelessly to get this over the line.
"The move [for Fury's team] was to speak [to Wilder's team] to see if there is a resolution [such as a step-aside fee for Wilder]. We can't control that.
"As far as I understand, Fury wants to fight Joshua and we had the deal to do so on August 14 in Saudi Arabia.
"We have to get our side in order, make sure we have our own plans. Hopefully they can resolve the issue and we can move forward with the August 14 fight.
"Over to them!
"They were wrong. That's on them. It's their responsibility, their problem. We hope they solve that problem but we have to look after Joshua."
Asked if he still expected Joshua's fight with Fury to go ahead on August 14 as planned, Hearn said: "I hope it does. We have grafted to make it happen and have a fantastic deal in place for a legacy fight and a huge amount of money, what both guys wanted.
"I hope. But am I hopeful? I don't know.
"We were told the arbitration wouldn't be a problem but obviously it is a problem.
"We have to think on our feet. We have to act accordingly. We still hope the fight can go ahead but it's out of our hands.
"The positivity from Team Fury was great for everybody. They had started training camps.
"[Joshua told me]: 'Take care of it, let me know, I'll be in the gym'.
"We know what we want - to win the undisputed championship and to fight Fury. But if his hands are tied we have to look elsewhere."