A source reported the news to Pro Football Talk's Mike Florio, who also posited that "it's hard to imagine Brown prevailing."
Brown's helmet was discontinued in 2011, per Florio, and the league could not re-certify it for 2019 because it is over 10 years old.
According to ESPN's Adam Schefter, the 10th-year wideout has threatened to walk away from the game if he's not allowed to use the helmet.
Schefter reported that Brown's concern with the new helmet is that it "protrudes out and interferes with his vision as he tries to catch the football."
As Florio mentioned, Brown would walk away from $30 million guaranteed if he did not play again. The wideout signed a three-year, $50 million deal this offseason.
However, Brown has insinuated leaving the game, as this Instagram post noted:
Brown is also suffering from extreme frostbite on his feet as this grievance process moves along.
There may be some good news on the horizon though. Per Michael Gehlken of the Las Vegas Review-Journal on Friday, a team official said Brown "is expected to rejoin the club at its training camp headquarters in the near future."
The foot injury "is largely considered a nonissue moving forward," per Gehlken.
Oakland opens its regular season on Monday, Sept. 9, against the Denver Broncos.