The San Diego Union-Tribune reported the 53-year-old Whisenhunt was returning to the job he held with the Chargers in 2013.
The Chargers finished with the NFL's fifth-ranked offense during Whisenhunt's year in San Diego.
The Chargers' offense has struggled in the two seasons since Whisenhunt left to become the Titans' head coach. He was fired on Nov. 3 after a 3-20 record as the Titans' coach, including 1-6 this season.
The Chargers last week fired offensive coordinator Frank Reich, who was Whisenhunt's successor.
The Chargers finished ninth in the league in total offense this season (371.8 yards per game), but ranked 26th with 20 points per game.