Brown released a statement following the ruling, in which he said he plans to play this season despite reportedly threatening to retire if he wasn't allowed to wear his preferred helmet.
It's "very unlikely" Brown will appeal the arbitrator's ruling, a source told ESPN's Adam Schefter.
If Brown doesn't wear a league-approved helmet this season, the Raiders will subject to league discipline, according to Kevin Seifert of ESPN.
The 31-year-old apparently will not be allowed to play or get paid if he refuses to adhere to the NFL's helmet policies, Tom Pelissero of NFL Network reports.
Brown filed a grievance in an attempt to continue wearing a 10-year-old helmet model which is no longer acceptable. His legal team reportedly threatened to hold the league liable if he suffered a head injury while wearing an approved model.
Despite the ruling, however, the NFL will allow the receiver to wear his preferred model if he finds one that fits and is less than 10 years old, a source told Pro Football Talk's Michael David Smith.
Along with this helmet issue, Brown has been dealing with foot issues following a cryogenic chamber mishap which reportedly left him with numerous blisters.