The Irish mixed martial arts star, 35, has been out of action since breaking his leg in a July 2021 defeat to Dustin Poirier but was scheduled to top the bill at UFC 303 on June 29.
It was announced earlier this month that he had been forced to withdraw as a result of an unspecified problem, which McGregor has now confirmed was suffered during training without full protective equipment.
McGregor said in a post on his official Instagram account that the injury was "gut-wrenching to take".
"We had a lapse in concentration and engaged in a training session without wearing the full protective gear and I hit the toe off the elbow and broke the toe clean. It needs a few weeks, that's it," he said.
"I couldn't justify to my team, or fans, that I make the walk hindered again. That walk has been seen. This next walk has got to be, and it will be, 100 percent Conor McGregor. The fans deserve it and we are getting close.
"A slight lapse in concentration and a nuisance of an injury was picked up. That's it. Take the lesson and move forward."
Light heavyweight champion Alex Pereira will defend his belt against Jiri Prochazka in the new main event in Las Vegas.
McGregor is one of the biggest stars in the history of the Ultimate Fighting Championship (UFC) mixed martial arts circuit, formerly holding the featherweight and lightweight titles.
He made headlines in November when he sent a series of social media posts during unrest in Ireland, criticising the government's immigration policy.