Eddie Alvarez felt he gave himself a "death sentence" in his UFC 205 loss to Conor McGregor on Saturday.
Featherweight champion McGregor made history by taking Alvarez's lightweight title off him with a second-round knockout in New York.
McGregor was superior from the opening bell at a sold-out Madison Square Garden, producing a masterclass with timing and precision to leave Alvarez rattled.
A crestfallen Alvarez posted an emotional Instagram on Tuesday addressing his defeat.
He wrote: "Congrats to Connor and his camp on an amazing accomplishment, these guys continue to deliver, hats off to you fellas.
"As for my performance, the only thing I can honestly say was I f****** blew it ... I did nothing I trained, I did the complete opposite of what we planned on a daily basis for 10 weeks.
"To sum up our plan in a sentence it was "Go left and mostly wrestle", instead I circled into his left hand and mostly boxed. Fighting the way I did was a for sure death sentence and the result was fitting.
"I say it all the time, there is really small margin for error at this level and I paid for my mistakes. I managed to make it to the biggest stage and audience in my long career and f***** it all up when I arrived.
"My heart sincerely hurts and when I dwell on it I fill up with regret and anger. If there was a list of what not to do against an opponent of this nature I did them all on Saturday."