Harris spent his last seven years in Chicago, and made the Pro Bowl each year from 2005-2007. He helped the Bears reach Super Bowl XLI before falling to Sanders' Colts. Indianapolis cut Harris on September 3 after inking him in August.
With Luis Castillo (tibia) and Jacques Cesaire (knee) out for an extended period of time, the Chargers hope that Harris can step in and provide reinforcement. San Diego is uncertain on Corey Liuget's (calf) status this week.
Sanders has played just 11 games since being named the NFL's top defensive player in 2007. He was released by the Colts in February following seven seasons with the club.
The 30-year-old appeared in only the 2010 season-opener against Houston before suffering a torn right bicep. He underwent surgery and was inactive for the next 11 games before the Colts finally placed him on injured reserve in December. He played two games for the Chargers this year and recorded 14 tackles before being sidelined.
The Colts selected Sanders with a second-round pick in the 2004 draft after a standout collegiate career at Iowa, and he has appeared in more than six regular-season games only twice.