Now we know why the Chargers brought back Charlie Whitehurst.

The San Diego Union-Tribune is reporting that the Chargers (who signed safety Atari Bigby on Friday night) cut longtime No. 2 quarterback Billy Volek. Whitehurst will now back up starter Philip Rivers. The paper reported the Chargers saved $750,000 in roster bonus by cutting Volek.

Denver, Kansas City and Oakland are all looking for a backup and the experienced Volek could interest all three.

Two years ago, San Diego traded Whitehurst, then its No. 3 quarterback, to Seattle in a deal that moved the Chargers up 20 spots in the second round of the 2011 draft. San Diego also received a third-round pick last season. Seattle had hoped Whitehurst was their quarterback of the future. Whitehurst started four games, threw 155 passes and had three touchdowns in Seattle.

Meanwhile, ESPN’s Adam Schefter reported that the Chargers signed Bigby, who formerly played for Green Bay and, most recently, Seattle. The six-year veteran has started 35 NFL games. The Chargers would like to draft a safety soon, but Bigby will be in the rotation. He replaces Steve Gregory, who signed with New England.

Meanwhile, San Diego running back Mike Tolbert is visiting another team Saturday and there is strong chance he will sign there. The Chargers could turn to Kansas City’s Jackie Battle as a backup if Tolbert leaves. Tolbert has also visited the Chiefs.