Colin Kaepernick is not the first athlete to come under fire for choosing protest over patriotism.
Others, such as Muhammad Ali and Mahmoud Abdul-Rauf, previously risked their careers to stand up for their religious beliefs.
"Abdul-Rauf of the Denver Nuggets, formerly Chris Jackson before converting to Islam, refused to acknowledge the (American) flag in protest, citing similar reasons as Kaepernick and saying that it conflicted with some of his Islamic beliefs," NFL Media reported.
The NBA player was suspended until he worked out a compromise with the league, which allowed him to pray quietly next to teammates as they listened to the anthem, the article noted.
Ali, who dominated the boxing world in the 1960s and 1970s, lost many fans in the early part of his career when he converted to Islam and began speaking out against U.S. foreign policy, Esquire reported.
He "famously refused to be drafted to protest the Vietnam War and American racism. He was sentenced to five years in prison, temporarily banned from boxing and fined $10,000," the article noted.
Ali spent some of his prime fighting years sidelined in the sporting world as he battled against the draft in court. It took him decades to repair his public image, but, when he died in June, Ali was hailed as a sports hero and praised for standing up for his beliefs.
"No athlete has had such a marked social impact since," The Huffington Post reported after his death.
When Kaepernick chose to sit during the national anthem at Friday night's preseason game, he called to mind Ali's sacrifices.
"So if you are criticizing Kaepernick now, but loved Ali, it just shows you really didn't care about what Ali stood for," tweeted Jemele Hill, an ESPN on-air personality.
Kaepernick has linked his activism to ongoing race-related protests, sharing articles about the Black Lives Matter movement on his Twitter feed.
"I am not going to stand up to show pride in a flag for a country that oppresses black people and people of color," he told NFL Media in an interview after the 49ers Friday night preseason game. "To me, this is bigger than football and it would be selfish on my part to look the other way."
The backlash against Kaepernick, already a second-string and much maligned football player, was swift. Former fans of the San Francisco 49ers quarterback criticized the player on social media. One man posted a video of himself setting his Kaepernick jersey on fire while listening to the national anthem, as Esquire reported.
It's unclear whether his actions will ever earn him the accolades Ali received, or if the controversy and his waning talent with speed the end of his football career.
The NFL released a statement Saturday noting that Kaepernick will not be punished for his actions. "Players are encouraged but not required to stand during the playing of the national anthem," said NFL spokesman Brian McCarthy, according to ESPN.
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