"Stefan Struve suffered a non-life-threatening, near-fainting spell backstage," the UFC said in a statement during the fight card. "Afterwards, the medical team did not feel he was fit to compete. With his health and safety in mind, he's been removed from the card and is currently under the care of the medical staff."
After the card, UFC president Dana White said Struve was "doing great" and had been cleared by doctors.
"He had something like a panic attack," he told the post-fight news conference. "He almost blacked out, he started hyper-ventilating, his blood pressure dropped. I don't know, just one of those weird things."
White said Struve had to ponder his fighting future.
"He needs to do some soul-searching. He's a super-talented guy. He's a great guy, we love the guy. I don't know, we'll see where he goes from here."
Struve, meanwhile, tweeted he was fine.
"Just want to let everybody know I'm OK, had a blackout in the locker room and we had to call the fight off, thanks for the love folks!"
White said both Struve and Mitrione would get their basic purse.
"I found out what happened and went to talk to Stefan," Mitrione said in a statement. "He was still in his chair and was very apologetic. He's such a competitor. Obviously health and safety come first."
The six-foot-11 Struve's MMA career appeared over last year when he was diagnosed with a rare heart condition.
Given the OK to resume fighting by his doctors in the Netherlands and the UFC's cardiologist, Struve (29-6) was looking forward to returning to the cage. Struve was also cleared by the Nevada State Athletic Commission.
"It looks good," Struve said in a pre-fight interview, "I just need to do checkups two times a year with my doctors in Holland and two times a year with my doctors in L.A.
"That's no problem for me. I actually like that I know for sure that my heart is healthy."
The 26-year-old Struve, ranked 12th among heavyweight contenders, last fought in March 2013 when he suffered a broken jaw in a knockout loss to hard-hitting Mark Hunt in Japan.
He returned to training some three months later. But he blacked out briefly while at his parents' home and went to hospital to be checked out. A stress ultrasound test, which uses high frequency sound waves to examine the heart's anatomy and function, found the heart issue.
Struve was diagnosed with a leaking aortic valve, as well an enlarged heart. It meant that his heart was only pumping 60 per cent of his blood into the aorta and then the rest of his body. The remaining 40 per cent was ending up back in the heart chamber.
Doctors used medication to control his blood pressure and the demands on his heart. Struve says he will probably have to undergo surgery at some point but hopes to do it after his fighting career.