It was a brutal punch that seemed to come out of nowhere.
He registered his own knockdown in the fifth round and had hit Marquez with a barrage of hard punches. The powerful Mexican counterpuncher had apparently suffered a broken nose.
In the sixth round, Pacquiao was taking it to his man. He was hitting him with lefts and rights, and you were left wondering whether Marquez would be able to survive much longer.
Then it happened.
In the final seconds of the sixth round, Pacquiao got too aggressive. While looking for his own knockout, he moved right into range and Marquez did not miss.
Marquez threw a blistering right hand punch that caught Pacquiao flush on the jaw. Pacquiao went down face first, and he was knocked out cold.
He was counted out and it took several moments for him to get up.
Pacquiao made a mistake and paid dearly for it. However, if he wants to continue with his career, it does not have to end with this knockout loss.
While Pacquiao suffered a brutal defeat, he fought with courage, skill and instincts against "Dinamita." Pacquiao won four of the first five rounds and appeared to be well on his way towards winning the sixth round before he got caught with the huge right hand. The sixth round could have been a 10-8 round for Pacquiao if he could have avoided that big punch in the final seconds of the round.
Pacquiao has many options in his life, and he is not beholden to continue his boxing career. He is a member of the Filipino House of Representatives, and he also has an interest in show business.
If he is fully committed to coming back and fighting again—his immediate reaction was to tell HBO's Larry Merchant that he wanted another shot at Marquez—he's certainly capable of returning.
It's something that he is going to have to think long and hard about in the harsh light of day. However, if he truly wants to get back into the ring and take a shot at a fifth fight with Marquez, the first five rounds and 2:57 of the sixth round show he can still handle himself.
It's those last three seconds he has to worry about.
It's certainly an error that can be corrected. He got too close to his powerful opponent, and he was in a vulnerable position.
But that does not mean that Pacquiao is finished. He can get back to training and take a bit more of a cautious attitude when he has the lead. He can still attack, but he must remember the referee's instructions that he forgot against Marquez:
"Protect yourself at all times."
If Pacquiao wants to fight again, and can make this one simple adjustment, he can salvage his legacy.
Now, he needs a rematch with Marquez. If Marquez gives him that opportunity, Pacquiao can turn this devastating knockout into another chapter in his biography.
It does not have to be the one that ends the story.