Charles Martin is "ready and waiting" to fight Kubrat Pulev if Anthony Joshua is unable to recover from a knee injury.
Joshua was pictured on crutches while attending a Black Lives Matter protest in Watford last week, with his left knee in a brace.
According to Boxing Scene, promoter Eddie Hearn confirmed the IBF, WBA and WBO world heavyweight champion will be out of action for four weeks after injuring himself when running.
Joshua is due to fight Pulev in October or November this year, but Martin, beaten by Joshua at the O2 Arena in 2016, is keeping a close eye on his recovery.
Leon Margules, the president of Martin's promoters Warriors Boxing, 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."