Davis, who was a restricted free agent, had been participating in optional workouts this month under an injury waiver. He fell to the turf Tuesday while backpedaling and grabbed the same knee in which he tore the anterior cruciate ligament in November.
Davis walked off the field, but he didn't return to the practice. A team spokesman said Davis was undergoing tests and the Panthers could know more Wednesday.
The Panthers didn't attempt to sign either player to a long-term deal. Davis and Marshall would have been unrestricted free agents under the old labor rules, but they became restricted free agents when the league and players failed to reach agreement on a new collective bargaining agreement, triggering an uncapped 2010 season.
The Associated Press contributed to this report.