Amir Khan may have only a month to agree to fight Kell Brook, or his rival will face American Errol Spence Jr. instead.
There has been much speculation in the UK about the British boxers meeting to contest Brook's IBF world welterweight title, and time is running out for the bout to be made for the spring.
But the eye-socket injury Brook (36-1, 25 KOs), 30, suffered in his last fight -- his defeat against Gennady Golovkin last May -- means it hasn't been ruled out because he needs more time to get ring ready.
Khan (31-4, 19 KOs), 30, has also had surgery since his Canelo Alvarez defeat last year. He hopes to return to action in April after recovering from an operation for a lingering problem with his right hand.
"Kell is back in training, but he's not able to spar yet, so we've requested a further four weeks from the IBF for negotiations," Brook's promoter, Eddie Hearn told ESPN.
"We're speaking to Khan. It could be Khan, if he wants it, but we can't promise that will happen. He wants a warm-up fight, and we want to go straight into it.
"Our priority is to listen to the IBF on who we have to fight, and they are telling us that will be Spence.
"I don't want to annoy the IBF about it. In 28 days we will start negotiations. It [Brook's next fight] will happen in May, and in the UK I expect. We've got 75 percent of the purse bid to play with, so I can't see us losing that."
Both Brook and Khan have expressed a desire to make the fight happen, but attempts to do so broke down last year, and the two welterweights stepped up two divisions to middleweight.