Cain Velasquez pulled out of his rematch with Fabricio Werdum at UFC 196. Stipe Miocic steps in as a substitute with about two weeks notice.

The oft-injured former heavyweight champion, fighting just six times in the last five years, cited back problems as the obstacle to competing. He offered an explanation on Facebook:

Training camp has been difficult for the last few weeks, but sometimes no matter how much the heart and mind want to fight through an injury, healing has its own timeline. We thought we’d get through it, but we had to face the truth that it wasn’t going to be OK in time to fight. All of us fighters have all fought injured at one time or another, but I can’t ignore this injury, as my doctor has indicated that not addressing the problems with my sciatic nerve could have longer-term consequences.

My fans deserve to see me compete as close to 100% as possible. I’m committed to giving them that. I’m not going to let my fans down, and I want to make sure I do the right things to ensure a long career.

Fabricio, I hope that you accept my sincere apology. I understand how you may think this is an excuse. From one professional to another, I can tell you I was committed and looking forward to competing with you again. I greatly respect you as a fighter and man, and we will meet soon.

The 20-5-1 Werdum enters the bout on a six-fight winning streak since rejoining the UFC. He boasts victories over Fedor Emelianenko, Alistair Overeem, Antonio Rodrigo Nogueira, and Mark Hunt in addition to Velasquez. Atop his well-established expertise in Brazilian jiu jitsu, Werdum has added outstanding boxing to his arsenal. His Ohioan opponent owns wins over Hunt, Andrei Arlovski, and Roy Nelson. Miocic, 14-2, also offers some of the best boxing among the UFC’s heavyweights.

“Something can always happen, so you never know,” Miocic told UFC.com. “I’m just super happy that I got the chance now. That’s what I’ve been working for, and I’m gonna take that belt. It’s mine.”