In a conference call with Bay Area reporters Sunday night, Cable delivered the potentially alarming news, saying Campbell had an MRI and X-rays and would be further evaluated this week.
The Raiders did not consider the stinger that knocked Campbell out of the preseason loss to the 49ers to be serious, Cable said.
The Raiders open the season in two weeks at Tennessee. It appears to be too early to project whether Campbell will be ready to play against the Titans.
If Campbell can't play, Bruce Gradkowski will get the call to open the season.
Campbell was taken off the field on a cart after being hurt on a sack by Travis LaBoy in the second quarter. He was down for about five minutes.
"Usually I recover pretty fast," Campbell said after the game. "Right now there's no timetable. The thing is just to improve and continue to keep getting better. We don't know the timetable or anything, but at the same time, I haven't really missed any time before."
Gradkowski was a sparkplug for Oakland in his preseason debut against San Francisco on Saturday night after Campbell was hurt. Gradkowski gave Oakland's struggling offense life last season when he took over for JaMarcus Russell.
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Still, Gradkowski has had trouble staying healthy.
Cable also said running back Michael Bush will undergo a procedure on his broken left thumb. Bush was still expected to be Oakland's starter this season, partly because he is more durable than Darren McFadden, who missed the first two preseason games with an injury, and Cable said Bush could play against the Titans.
The injury outbreak to Oakland's skill-position players extends to No. 1 receiver Chaz Schilens, who will return from the arthroscopic knee surgery he had last week in three-six weeks. That makes Schilens questionable for the season opener, which will be played 19 days after his procedure.
In still more injury news, center Samson Satele will return to practice Monday after missing the past week with a sprained ankle. Rookie Jared Veldheer got mixed reviews from Cable for his performance against the Niners.
"I saw a guy playing center for the first time in this league," Cable said. "You got to learn where to put his helmet a little bit more consistently. When he did, it was pretty good stuff. So, there's a lot to grow on. It was actually a pretty good effort by him."
Veldheer will work at both left tackle and center this week, where he is expected to be the backup at both positions.
Cornerback Chris Johnson is also expected to practice after missing the past two games with a hamstring injury. Johnson, who was limited in practice last week, is competing with Stanford Routt for the starting spot across from Pro Bowler Nnamdi Asomugha.
Defensive tackle Desmond Bryant (hyperextended elbow) and cornerback Joey Thomas (concussion) are day to day with injuries sustained Saturday. Defensive linemen Jason Richardson (knee) and Alex Daniels (hamstring) will likely not play in the exhibition finale Thursday against Seattle.
Information from ESPN.com's Billl Williamson and The Associated Press was used in this report.
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