Former quarterback Colin Kaepernick has filed a collusion grievance against NFL owners.
Kaepernick has not been able to find another job in the NFL since opting out of his San Francisco 49ers contract earlier this year.
The 29-year-old has hired attorney Mark Geragos to assist him.
Kaepernick became a controversial figure last year when he became the first player to refuse to stand for the national anthem as a sign of protest against what he called racial injustice in the United States.
Many other players have since followed his lead, leading to fan protests and concern among owners and NFL commissioner Roger Goodell that the actions were hurting TV ratings and attendance.
The NFL players' union said Sunday it will support Kaepernick but will have no role in the grievance. The union did not learn of it until it was reported in the media.
"Our union has a duty to assist Mr. Kaepernick as we do all players and we will support him," the statement said.
"The NFLPA has been in regular contact with Mr. Kaepernick’s representatives for the past year about his options and our union agreed to follow the direction of his advisors throughout that time.
"We first learned through media reports today that Mr. Kaepernick filed a grievance claiming collusion through our arbitration system and is represented by his own counsel.
"We learned that the NFL was informed of his intention to file this grievance before today. We are scheduling a call with his advisors for early this week."