Whitehurst agreed to a two-year contract Friday to return to the Chargers. A short time later, the Chargers released Volek.
Whitehurst's deal is worth $4 million and includes a $1 million signing bonus, a league source told ESPN NFL Insider Adam Schefter.
Whitehurst was San Diego's third-string quarterback behind Rivers and Volek from 2006-2009, and never attempted a pass in the regular season.
Whitehurst was traded to Seattle and played in nine games in two seasons, throwing for 805 yards and three touchdowns, with four interceptions.
On Jan. 2, 2011, Whitehurst started Week 17 and helped the Seahawks beat St. Louis 10-6 to win the NFC West. He threw for 192 yards and a touchdown.
The 35-year-old Volek has been in the NFL since 2000 with Tennessee and San Diego and has appeared in 39 games, starting 10.
Volek's most memorable appearance with the Chargers was in January 2008 when relieved the injured Rivers in the fourth quarter of a divisional playoff game at Indianapolis and led a game-winning, eight-play, 78-yard drive that he capped with a one-yard sneak.
Volek appeared in only two games last season, with no attempts. In 2010, he appeared in three games and completed one pass for eight yards.
Neither general manager A.J. Smith nor coach Norv Turner returned calls seeking comment.
The Chargers also agreed to a two-year contract with veteran safety Atari Bigby, who had 18 tackles and a sack in 15 games as a reserve for Seattle last season.
Bigby's deal is worth between $2.5 and $4.5 million, a league source told Schefter.
Bigby has played in 61 NFL games, starting 27 for Green Bay in 2007-09.
The Chargers expect third-year safety Darrell Stuckey to compete with Bigby at strong safety.
Information from The Associated Press was used in this report.