Andy Ruiz Jr said three months of partying and a lack of proper training left him “overweight” for his world title rematch with Anthony Joshua.
Ruiz, 30, was comprehensively out-pointed in Saudi Arabia, six months after shocking the boxing world by stopping Joshua in New York.
The Mexican vowed he would be “a lot better” if he gets a third fight.
“I should have trained harder. I should have listened to my team and coaches,” he said.
“For this fight I was overweight. I didn’t move how I wanted to.”
He appeared regretful and apologised to his trainer Manny Robles and father Andres at his post-fight news conference but was applauded out at its conclusion.
The former champion claimed he had initially lost weight after his win but added: “I put on the weight and it was my mistake.
No-one knows about the training we had. We did the best we can.
“Being a champion for the first time, being over here and there and missing training, it was hard.
“For the trilogy I will be a lot better. I want to redeem myself in Saudi Arabia.”
“I don’t want to say the three months of partying or celebrating didn’t affect me, as to tell you the truth it kind of did,” Ruiz added.
“There’s no excuses. The partying got the best of me. The next fight is going to be a lot different.
“People are still doubting me. I am still a dangerous fighter to anyone in the division. At percent, I can dominate anyone in the world.”