Santos tore the ACL, MCL, PCL and meniscus of his left knee in the UFC 239 main event Saturday night in Las Vegas, his manager Alex Davis confirmed with ESPN. Tests came back with that information Monday, Davis said.
Brazilian outlet Combate was the first to report the news. Davis said that Santos is likely to be out for the rest of 2019 and beyond.
Santos also has a partial ligament tear in his right knee, Santos' coach, Phillip Lima, told ESPN Brazil.
Jones won the fight by split decision to retain the 205-pound title, but Santos fought extremely well in defeat. It was the first time Jones had ever gone to decision and lost on one of the judge's scorecards.
There are four primary ligaments that stabilize the knee, and Santos tore three of them, plus the meniscus, in his left knee. How he remained standing let alone able to challenge Jones through five rounds to a split decision, all while battling pain and instability, defies logic.
The full extent of the damage won't be truly appreciated until he is in the operating room and his surgeon has an opportunity to visualize the knee. But if there is some good news for Santos, it's that he has been fully integrated with the staff at the UFC Performance Institute for some time and it already has a plan in place for his post-operative rehab.
Santos said in the UFC 239 postfight media conference that he felt something give way in his left knee at the end of the first round. By the fourth and fifth rounds, Santos was clearly favoring the leg, but still fighting effectively against Jones, who is regarded as perhaps the best pound-for-pound MMA fighter of all time.
Two judges actually had Santos winning the fourth round and one had him winning the fifth. Some felt Santos had even done enough to win in a close decision.
"I left everything in there even after injuring my knee in the first round," Santos said at the media conference, through an interpreter. "I fought four rounds on a southpaw stance, unable to move like I usually move, unable to throw the strikes I usually throw. I didn't shock the world because of those circumstances, but I did what I could do in those conditions and I'm satisfied."
Jones said afterward that he was not aware that Santos had an injured leg during the fight. He acknowledged that Santos made him look "bad" during the bout and used kicks to Jones' calf as a key technique.
"Boy, was he tough," Jones said. "We all knew Thiago's best chance was to knock me out. I played it smart and brought home this gold for my family and team. He was technically a lot more sound than I thought. Thiago Santos is a black belt in Muay Thai. I'm proud of myself, because I stood with a guy who's been kickboxing way longer than me."
Santos (21-7) had won four straight coming into the bout, including the past three since moving up from middleweight to light heavyweight. In those three bouts, Santos won by TKO or knockout. Santos, 35, has won eight of 10 fights overall.
ESPN's Stephania Bell contributed to this report.