Duffee, 31, describes it as the worst loss of his career. According to him, just cognitively recovering from a knockout loss like that can take several weeks. There's a feeling of getting mentally "reorganized" after that level of trauma, not to mention a strong sense of letting others down.
"I just remember kind of isolating myself," Duffee told ESPN's 5ive Rounds podcast. "It's brutal. It's demoralizing."
But for Duffee, the hardest part of dealing with that type of defeat was not knowing if he could afford to fight again.
"Me losing to Frank was not necessarily the ordeal," Duffee said. "It was more like, 'Oh s---, how do I get to keep doing this?'
"I just made $10,500. After they take out taxes, that's the check I got cut. Your family is not behind you at that point. Your friends are like, 'OK, so your career is over.' That's how I felt."
Duffee (9-3) has not fought since the loss to Mir, but he recently signed a contract extension with the UFC. He is scheduled to fight Mark Godbeer at UFC 209 on March 4.
His disclosed fight purse for the bout against Mir in July 2015 was $12,000. After taxes and training expenses, Duffee estimates he netted between $6,000 and $8,000.
The competitor in him wanted to return to action as soon as possible. Move on from the loss. From a practical standpoint though, Duffee couldn't get himself to start another camp.
"Business-wise, you're paying to play at that point," Duffee said. "I was making [$12,000 to show, $12,000 to win]. You've got to take time off work to focus on the task at hand, because you're going to fight another man in a cage. You're not going to take that lightly. You're not going to play with that. That could have long-term effects on your life.
"So you have to take time off work, and you're paying to play. The heavyweight division is a gamble, right? There's no question about it. You don't see heavyweights rattling off huge win streaks.
"If you come out [with a loss], you just took two or three months off work, and you might get paid $8,000. If you come out a winner, you might get $16,000. And what that takes away from your family, work, life -- it's a big undertaking. I didn't feel good about it."
Duffee had one fight left on his old deal when he signed an extension last week. By negotiating that extension, Duffee receives an immediate increase in pay for his next fight. However, doing so also prevents him from testing free agency, which a growing number of fighters are using to their advantage.
Although he admits he might have left some money on the table by signing the extension, Duffee is content with his decision to re-sign. There are additional reasons he says he'd want to stay in the UFC.
"I want to fight the best," Duffee said. "I want to test my skill set. With the UFC, right now, the best talent is here. No question.
"And this might sound crazy, but USADA [United States Anti-Doping Agency, the UFC's anti-doping program partner] -- you don't have that anywhere else. This is a real league. Top to bottom, it's an organization that makes things easy. They took great care of me after my fight with Mir."
In 2017, Duffee is hopeful he can accomplish something he has failed to do since 2007: fight three times in a calendar year.
He has been a professional fighter for 10 years -- 12 pro fights, across five countries in seven organizations. He has fought two of the best heavyweights of his era in Mir and Alistair Overeem and headlined a televised event.
All that said, Duffee feels he has never really had a career in mixed martial arts. Perhaps this is the year that finally changes.
"If I go out and win my fight, yeah, it's a career. No question," Duffee said. "If I go out and lose, no. I think a big factor with heavyweights is that if you lose, you're hurt. I'd walk away with three to four months [worth of money] to live on, and I'd have to figure out what I could do.
"If I win, yeah, I have a career. I'd be turning around and fighting again, making decent money. It's all a gamble."