A little-known rule from 2010 has temporarily kept Panthers linebacker Thomas Davis from being a free agent this year, according to a source. Davis, considered one of the most talented linebackers in this year's free-agent market, spent last season on the physically-unable-to-perform list because of a knee reconstruction. Because he missed the season, his $3.268 million salary from 2010 passed to this season and put him under contract for the 2011 season, according to the source said.