This grievance is the latest part of Brown's ongoing fight against an NFL policy that requires helmet certification according to industry standards. Brown already lost a grievance against the NFL that sought an exception so he could wear his Schutt Air Advantage, a helmet he has worn throughout his career.
A number of players, including New England Patriots quarterback Tom Brady, were given a one-year grace period last season that allowed them to wear helmets that were recently banned. The NFL and NFL Players Association eliminated the grace period after last season.
Brown believed last week that he had found a suitable replacement -- a helmet manufactured within the past 10 years -- but it did not meet additional testing parameters. Because the replacement helmet had just been banned, Brown believes he is owed the same grace period that other players were given last season, when their helmets were tested and banned, a source told Schefter.
Brown's agent, Drew Rosenhaus, said Monday that the receiver's helmet concerns are safety related.
General manager Mike Mayock said Sunday that Brown had left camp over the helmet issue, and it was "time for him to be all-in or all-out." Rosenhaus disputed that Brown had left camp, and the wideout was back with the team at the facility Monday morning, a source told Schefter.
The Raiders acquired Brown in a trade with the Pittsburgh Steelers in March, then signed him to a three-year deal worth $30.125 guaranteed.
ESPN's Paul Gutierrez and Kevin Seifert contributed to this report.
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