Per The Athletic's Vic Tafur on Saturday, the star wideout is meeting with a foot specialist on Saturday.
The Raiders have been coy about Brown, who hasn't been a full participant in training camp since the team reported on July 26.
"I think we're all disappointed," head coach Jon Gruden said about Brown's absence on Friday, per The Athletic's Lindsay Jones. "We want to get the party started."
Following a brief stint on the non-football injury list, Oakland's medical staff cleared Brown to practice July 28.
Brown has maintained relatively good health throughout his career, though he has had to miss the occasional game because of injury. He missed the divisional round in the 2016 postseason with a concussion, and he missed the final two contests of the 2017 regular season because of a torn calf muscle.
He also dealt with a knee injury near the end of the 2018 campaign.
Before he forced his way out of Pittsburgh, Brown had become arguably the most productive receiver in football. The seven-time Pro Bowler recorded six consecutive seasons with at least 1,200 yards and eight touchdowns while catching passes from Steelers quarterback Ben Roethlisberger.
Following a tumultuous 2018 season, though, Brown decided it was time for a change. Now, he is tasked with proving a change of scenery won't affect his numbers.
Brown was the biggest splash of the Raiders' busy offseason. Losing him for any amount of time would make quarterback Derek Carr's job that much tougher, with the likes of Tyrell Williams and rookie Hunter Renfrow in line for increased usage.