"The Monster Among Men" posted a heartfelt message on his Instagram account, wishing Reigns a speedy recovery.

"I've been in many wars with this man. He's been my biggest rival on tv!!!! But behind the scenes he's one of my closest friends!!!! He's is one of the toughest men I've ever met and I have no doubt he'll kick leukemia's ass and and be back to battling with me soon and I can't wait. You and your family are in my prayers. Love you Big UCE!!!!! #FUCancer"

This is not the first time Strowman opened up about his relationship with Reigns off screen. In the past, Strowman talked about his camaraderie with Roman behind the scenes, and heaped praises toward him.

"Roman's work ethic is just like mine. We go out every night, we give 100 percent of what we have to go, we put our bodies on the line and we know that, at the end of the night, we're going to have you on your feet, you're going to be hoarse, and you'll be going crazy because we did our job entertaining you. From top to bottom, we're the best athletes in the world."