Kirby Lee-USA TODAY Sports

New Oakland Raiders wide receiver Antonio Brown started training camp on the non-football injury list. Since being activated, he’s taken part in a pre-practice walk through this past Sunday and the early stages of practice Tuesday. That’s it.

Oakland is being tight-lipped about what is ailing the recently acquired Pro Bowler. But in speaking about Brown following Friday’s practice Raiders head coach Jon Gruden seemed a bit frustrated.

“I think we’re all disappointed. We think he’s disappointed,” Gruden told the media, via ESPN’s Paul Gutierrez. “We’d like to get the party started. We’d like to get him out here. He’s a big part of the team.”

Acquired from the Pittsburgh Steelers following a drama-filled 2018 campaign, Oakland is relying on Brown to be the No. 1 receiver for struggling quarterback Derek Carr.

Now, more than a week into camp, Brown remains sidelined to a mysterious injury. It can’t be seen as great news for the Raiders.