After a prolonged battle in hopes of being able to wear his old helmet, Brown will have to wear an NFL-approved helmet this season -- barring a long-shot legal challenge, Pelissero added.
If Brown refuses to wear an NFL-mandated helmet, he will be unable to play or get a paycheck.
Mum on the helmet issue since it came to light on Friday, Brown issued a statement via Twitter on Monday following news of ruling.
"While I disagree with the arbitrator's decision, I'm working on getting back to full health and looking forward to rejoining my teammates on the field," he wrote. "I'm excited about this season appreciate all the concerns about my feet!"
NFL Network's Michael Silver reported Friday, in addition to a seemingly unprecedented NFL injury of frostbitten feet, Brown's absence from Raiders practices was due in large part to his frustration to having to wear an NFL-regulated helmet rather than the one he has worn for the majority of his career.
"The helmet thing is a personal matter to him," Gruden said. "He has a strong feeling about what he's worn on his head and we're supporting him. We understand the league's position as well, so we're in a tough spot. And we hope Antonio is back here soon, because he's exciting to be around."