Anthony Joshua's knee problem is being monitored by Charles Martin who is next in line to fight Kubrat Pulev for the IBF belt, if Britain's unified champion is ruled out through injury.
Joshua has played down fitness concerns after being pictured on crutches with a brace on his left knee, with promoter Eddie Hearn suggesting he will be sidelined for at least four weeks ahead of his next world heavyweight title fight against Pulev, the IBF's mandatory challenger.
But American Martin, who is ranked at No 2 by the IBF, is keeping a close eye on Joshua's condition as he would be next in line to face Pulev for the IBF belt, if Joshua was not fit enough to fulfil his title defence.
Leon Margules, President of Warriors Boxing, who promote Martin, told Sky Sports: "If Joshua is injured and can't defend on a mandatory basis, and the title becomes vacant, we'll fight Pulev for the vacant title.
"Charles Martin is fit, ready and waiting for that opportunity. We've been waiting for the opportunity for a long time.
"We were actually reading all about Joshua-Fury, and then thinking to ourselves that the IBF would then vacate the IBF title, because he didn't do his mandatory. He's already had exceptions. We would then demand the IBF vacate the title and allow Pulev to fight for Joshua's vacant title."
Joshua took the IBF title from Martin with a second-round stoppage win in 2016, but 'Prince Charles' has won five of his next six fights, including a knockout win over Gerald Washington in an IBF eliminator in February.
Asked whether the IBF would enforce Pulev against Martin, Margules told Sky Sports: "That's for the IBF, but the rules do not give a medical exception for a mandatory.
"I think Charles could beat Pulev. I think Charles could beat Joshua if they fought again. I think Charles is a totally different fighter today than he was when he went over and fought Joshua."