The N.F.L. Players Association filed a grievance against the league on behalf of Eric Reid on Monday.
Reid, the free agent safety who has knelt during the national anthem at games for the past two seasons, had filed a grievance of his own last week against the N.F.L., saying the league had blackballed him because of his actions. Released by the 49ers, he has yet to find a new team.
The former 49ers quarterback Colin Kaepernick, who led a wave of sideline demonstrations against social injustice and police brutality against African-Americans, filed a similar grievance last season.