Colin Kaepernick's former team-mate Alex Boone described the San Francisco 49ers quarterback's decision not to stand for the United States national anthem as "shameful" and said he would have confronted him on the sideline.
Kaepernick remained seated during the national anthem in the 49ers' preseason meeting with the Green Bay Packers last Friday to protest against oppression "of black people and people of colour".
His actions have sparked widespread debate, with many NFL players supporting Kaepernick's right to freedom of speech while others have condemned his decision.
Boone, who played with Kaepernick for five seasons with the 49ers, falls in the latter camp.
The Minnesota Vikings guard, whose brother served with the Marines, said: "We probably would have had a problem on the sideline.
"You should have some f****** respect for people who served, especially people that lost their life to protect our freedom.
"We're out here playing a game, making millions of dollars. People are losing their life, and you don't have the common courtesy to do that. That just drove me nuts."
Boone's words are in stark contrast to those of another former 49er, wide receiver Anquan Boldin, who supported Kaepernick's right to protest.
"Anquan said it this morning [that Kaepernick had the right to protest], and I was kind of mad that he said it, because I was like, 'God, if Anquan says it, I've got to go with it', because Anquan was such a leader, and he's a smart guy," Boone added.
"He gets it. But at the same time, I was upset with [Kaepernick's] choice to do that and kind of disrespect the flag."