Pacquiao has a lot on the line, too—a second straight loss would be devastating to his legacy and puts a damper on any speculation surrounding a fight with Floyd Mayweather. However, it's Marquez that can define his entire career.
The pressure for Marquez to win is immense. Despite going toe-to-toe with Pacquiao in three classic bouts with the pound-for-pound great, he's never once come out on top with his hand raised at the end of the night.
Sure, he probably deserved one of the decisions along the way, but the official ledger still reads 0-2-1. In the end, it will look bad for his legacy if he goes 0-3-1 or even 0-2-2 against his greatest rival.
In terms of legacy, Marquez has long been an underrated boxer. The Mexican fighter has won seven world titles in four different weight divisions and has been a mainstay in the pound-for-pound rankings, yet he's been overshadowed by Pacquiao and Floyd Mayweather his entire career.
Some of that overshadowing is warranted. Marquez had his chance against both men and came up short, once against Mayweather and three times against Pacquiao.
However, a win over Pacquiao would change the way that fans look at his career. A knockout against Pacquiao would make Marquez a legend.
You can argue about Pacquiao's place in history all day. In the end, we can say for sure that he was one of the greats of his generation. He may not be perfect, but he's one of the most accomplished boxers in history and dominated the current landscape.
Despite Marquez's accomplishments in the ring, he doesn't have that signature win that boxing fans will remember him by. Outside of a unanimous decision victory over Marco Antonio Barrera, his victories have largely come against opponents that the casual boxing fan isn't very familiar with.
A knockout win would change all of that. Rather than going to the scorecards and hoping to win a decision that isn't marred by controversy, a knockout win for Marquez would prove that he is capable of unequivocally being the better man.
After a series full of controversy that has had many fans feeling Marquez has been robbed of victories by the judges, Marquez could show that those fans are right. A victory by knockout has to cast a different light on the previously recorded decisions.
If Marquez can win by knockout, it not only alters the way that this rivalry is viewed, it would push Marquez's legacy to the next echelon of fighters.