Franklin, who left Baltimore to sign with San Francisco in the offseason, was injured when two offensive linemen rolled onto his leg during a play. He fell facedown, then rolled over holding his left knee while trainers huddled around him.

Coach Mike Nolan said Franklin sprained his knee but will undergo more tests.

"The fear would be it would be something other than a sprain," said Nolan, who coached Franklin when both were with the Ravens. "It would be a piece of adversity, something we would need to do a good job responding to."

Slated to start in the middle of the 49ers' 3-4 defensive scheme, Franklin's absence could be a potentially serious blow to the club's season.

Defensive lineman Ronald Fields, who might fill in for Franklin along with Isaac Sopoaga, spoke with Franklin after the injury.