"What NFL owners did today was thwart the players' constitutional rights to express themselves and use our platform to draw attention to social injustices like racial inequality in our country," Jenkins wrote. "Everyone loses when voices are stifled."
Jenkins, who is one of the leaders of the Players Coalition, added that he won't let the updated policy change how he tries to affect social change.
"While I disagree with this decision, I will not let it silence me or stop me from fighting," he wrote. "The national conversation around race in America that NFL players forced over the past two years will persist as we continue to use our voices, our time and our money to create a more fair and just criminal justice system, end police brutality and foster better educational and economic opportunities for communities of color and those struggling in this country.
"For me, this has never been about taking a knee, raising a fist or anyone's patriotism but doing what we can to effect real change for real people. #thefightcontinues."
On Wednesday, NFL owners approved a national anthem policy for the 2018 season. According to the rule, players are required to stand for the anthem if they are on the field. If they choose not to, their team will be fined by the league.
The policy also stipulates players may choose to stay in the locker room "or in a similar location off the field" until the anthem has been completed.
"The policy adopted today was approved in concert with the NFL's ongoing commitment to local communities and our country—one that is extraordinary in its scope, resources, and alignment with our players," commissioner Roger Goodell said in a statement, per NFL.com's Austin Knoblauch.
"We are dedicated to continuing our collaboration with players to advance the goals of justice and fairness in all corners of our society. It was unfortunate that on-field protests created a false perception among many that thousands of NFL players were unpatriotic. This is not and was never the case."
Goodell's announcement came one day after the NFL and the Players Coalition finalized a $90 million partnership to fund social justice initiatives in local communities, according to NFL Media's Jim Trotter.