Brown began training camp on the non-football injury list due to an undisclosed injury. According to ESPN, the All-Pro wideout was suffering from sore feet.
After he was activated from the NFI list, the Pro Bowl pass-catcher participated in a pre-practice walk-through July 28. Brown was limited in practice Tuesday, later showing a picture of his blistered and peeling feet on Instagram.
The Raiders were off Wednesday, and Brown wasn't seen on the practice field for the remainder of this week.
"I think we're all disappointed," Raiders head coach Jon Gruden said. "We think he's disappointed. We'd like to get the party started. We'd like to get him out here. He's a big part of the team.
"I want the guy out here as soon as possible. I'd like him to never leave and stay in the huddle every play. But life goes on and you've got to continue to work and the other guys got to take advantage of these opportunities, and so far, they have."
Brown recorded 104 catches for 1,297 yards and a career-high 15 touchdowns with the Pittsburgh Steelers last season. The Steelers traded him to the Raiders this off-season.