San Francisco linebacker Travis LaBoy beat left tackle Mario Henderson on the play and drove Campbell in the ground.
Campbell, who landed hard on his right shoulder, was down on the ground for about five minutes before walking gingerly to the cart.
The Raiders said Campbell had a stinger.
"It felt a lot worse at the time that it happened, but ... they don't think it's going to be really, really serious," Campbell said after the Raiders' 28-24 loss. "But at the same time it's something we've got to take care of before the season starts."
Raiders coach Tom Cable told the San Jose Mercury News he expects Campbell to be ready for the season opener on Sept. 12 at Tennessee.
"Yeah, I do [expect him to be ready]," Cable told the Mercury News. "At this point, it does not seem to be a serious issue."
Before leaving, Campbell got off to another strong start, completing four passes for 67 yards on an 81-yard touchdown drive. Darrius Heyward-Bey had two of his three catches on the drive, fullback Marcel Reece had a 34-yard reception and Michael Bush scored on a 1-yard run. Campbell finished 6-for-8 for 93 yards.
Bruce Gradkowski replaced Campbell to make his first appearance of the preseason.
Bush suffered a broken left thumb in the second half. He told the Mercury News he hopes to play in the opener.
"I have no idea yet," Bush said when asked if he'll miss any action. "I'll find out Sunday."
Information from The Associated Press contributed to this report.