John Kavanagh reveals Conor McGregor injured knee before UFC 189 after sparring with Rory MacDonald

Conor McGregor is undoubtedly one of the very best fighters in mixed martial arts (MMA) today. But even beyond his natural skill set, the Irishman is as tough as they come.

Leading into his UFC 189 showdown with then Ultimate Fighting Championship (UFC) Featherweight titleholder Jose Aldo, McGregor hinted that he was fighting through injury. Aldo would later pull out of the title fight, Chad Mendes would replace him, and McGregor would go on to defeat "Money" via second-round TKO to win the interim strap.

Still, the MMA world has always been curious as to how injured McGregor truly was. To fight Mendes on short notice is a task in itself, especially if you're not 100 percent healthy.

In a recent report by The 42 (h/t FOX Sports), McGregor's head coach, John Kavanagh, details the injury in his new autobiography, "Win or Learn."

"Artem [Lobov] explained that Conor and Rory MacDonald -- a Canadian welterweight who was scheduled to take on the champion, Robbie Lawler, in UFC 189 -- had been training together that afternoon," wrote Kavanagh. "Nothing strenuous, just a bit of grappling. But during their session, Rory landed awkwardly on Conor's left knee -- the same knee he had injured against Max Holloway."

Kavanagh is eluding to McGregor's 2013 meeting with Holloway, which is the only time in the Irishman's career that he's been taken the distance, winning via unanimous decision.

"By now, Conor had mentioned the injury to Dana White, who recommended that Conor visit a clinic in Germany which specialised in stem-cell therapy," added Kavanagh regarding the build up to UFC 189. "As a sufferer of Ménière's disease, Dana had undergone stem-cell treatment at the same clinic and found it to be very effective. Conor took his advice."

"I still wanted Conor to have a scan. If we needed to push the fight back a few months, so be it. But he was adamant. There was no telling him otherwise. All I could do was take his word for it and devote myself to the task of preparing him accordingly."

There you have it. It seems as if Rory MacDonald had something to do with McGregor's injury leading into his UFC 189 title fight. Still, "Notorious" pushed through the pain, refused to reveal the physical ailment to the masses, and proceeded to dispose of Mendes in short time.

Currently scheduled to take on Nate Diaz in a Welterweight rematch at UFC 202 on Aug. 20, McGregor will try to stay healthy once again leading into the biggest fight of his MMA career.