Antonio Rodrigo Nogueira suffered back-to-back losses for the first time in 15 years as a professional MMA fighter, and he won’t be able to return to the Octagon this year.

"Minotauro" tore his right ACL days before his UFC Fight Night 39 main event against Roy Nelson, but decided to fight with the injury. The bout lasted less than four minutes, as Nelson knocked Nogueira out after rocking him several times.

The former PRIDE and UFC heavyweight champion underwent surgery Tuesday afternoon in Rio de Janeiro, Brazil, with doctors Marco Aurelio Sousa and Marcio Tannure, and he’s expected to be back in action in six months.

"Rodrigo has an ACL injury, which is a ligament that stabilizes the right knee. He did a reconstruction of this ligament," Dr. Tannure stated in a release. "The procedure was a success, everything went as expected, and we expect him to fight again in approximately six months."

According to Tannure, "Minotauro" should have canceled his fight because of the knee injury.

"Since it’s a disabling injury, the ideal would be not fighting. Besides the pain, is creates instability since the knee pops out all the time, so it limits your movements," he said. "He’ll need to go through a physical therapy work and conditioning."

"Minotauro" entered the UFC in 2006 with a 29-4-1 (1 no-contest) record, but went 5-5 inside the Octagon. The legendary heavyweight, who had never been finished before his run in the UFC, suffered all of his recent losses via knockout or submission.