Rosenhaus said on 7 Sports Xtra on Sunday:
"We have worked very closely with the Raiders and will continue to work very closely with the Raiders. I'm not sure we agree that we've exhausted all the options, as Mike Mayock said, but there's no doubt it's still an ongoing process. We are trying to work with the team and the league and the union on a solution. We haven't figured that out yet.
"To say that AB is upset about the decision to not let him wear his helmet is accurate, but we're still processing it and figuring it out. I wouldn't make too much about him not being there today, as much as we're still trying to come up with a solution that works for everyone."
Rosenhaus later appeared on ESPN's Get Up! on Monday morning, saying he thinks Brown will be "back soon" but acknowledging that this is a "major issue" for the Pro Bowler.
"Antonio is very genuine. He wants to be there, he wants to be part of the team, he wants to practice," Rosenhaus said. "But he'd also like to do it with the helmet he's worn his entire life...this is his life. He's risking everything. He's got a family, he's had a concussion before. This helmet has kept him safe. He's had brutal hits, and we're just trying to find a way to work it out."
Brown was absent from practice Sunday after the NFL ruled he could not wear any version of the Schutt Air Advantage he's worn his entire career. The league initially ruled Brown could wear a version of the helmet made in the last 10 years, but Pro Football Talk reported the NFL changed its position after AB found a 2014 iteration.
According to the report, the NFL required safety testing for the helmet, which it failed. Rosenhaus said Brown has tried around 15 different approved helmets, none of which are to his liking.
"We've been talking to helmet companies, trying to get this straightened out, trying to find a helmet that will work, trying to get his helmet approved, trying different helmets. Antonio's probably worn 15 different helmets to try to find something similar to what he currently has. We've been unable to do that, but we're going to continue to work," Rosenhaus said.
"I called Mike late last night and said, 'Let's get every available helmet that's approved and get every one of them out. Let's find a solution.' We are working with the team to get this done. I don't expect this to be a long-term issue. I'm working with Antonio, I'm working with the team, I'm working with the league, the union, everybody that you can think of to get this hammered out."
Mayock expressed frustration at the situation after Brown no-showed Sunday's practice over displeasure with the league's decision:
"Antonio Brown is not here today, and here's the bottom line: He's upset about the helmet issue. We have supported that. We appreciate that. We have, at this point, exhausted all avenues of relief. From our perspective, it's time for him to be all in or all out.
"We're hoping he is back soon. We have 89 guys busting their tails. We are really excited about where this franchise is going, and we hope AB is going to be a big part of it starting Week 1 against Denver. End of story. No questions."
Brown has been intermittently absent during a large chunk of Raiders camp because of frostbite in his feet and displeasure over the helmet situation. The league banned his Schutt Air Advantage helmet beginning this season because it is not approved by the National Operating Committee on Standards for Athletic Equipment.
In a grievance hearing, Brown threatened to hold the league liable if he suffers a head injury wearing a league-approved helmet, per Daniel Kaplan of The Athletic. The NFL won the grievance but initially said Brown could wear a Schutt Air Advantage helmet made within the last 10 years, and the wideout returned to practice.
After the helmet failed inspection, Brown took to Twitter to express his anger (NSFW language):
NFL Network's Ian Rapoport reported Monday that Brown is having the league test two additional versions of the Schutt Air Advantage that did not undergo testing last week.
By showing their frustration in such a public manner, it's clear the Raiders have reached the end of their rope with this controversy. They've already gone through the first two weeks of the preseason without Brown in the lineup, and he's missed important reps in practice that could affect his chemistry with Derek Carr.
But having already given Brown a massive new contract this offseason (three years, $50 million), the Raiders' potential acts of recourse here are minimal. Suspending or fining him will only further alienate him from the team, and sending him the so-called five-day letter puts him at risk of missing the entire season.