Coach Marty Schottenheimer chose Brees as his starter in a tightly-contested competition with 39-year-old Doug Flutie.
"Drew Brees has earned the chance to play and get better," said Schottenheimer, who a few days ago said the competition was too close to call. "This gives Drew the opportunity to participate as the starter in the final two preseason games. He's 23 years old and will improve with every snap."
Brees was a second-round choice in the 2001 draft out of Purdue and completed 15-of-27 passes for 221 yards and one score in his only appearance with the Chargers.
Content that Brees is ready to take the next step, Schottenheimer bypassed Flutie, who has thrown for 13,249 yards in eight NFL seasons.
"Doug is better prepared and ideal for coming off the bench because of his mobility," Schottenheimer said. "He can come off the bench and create tremendous problems for the opponent's
Going with a new quarterback is not new for Schottenheimer, who, as coach of the Cleveland Browns, went with rookie Bernie Kosar in 1985.
The Chargers open their season at Cincinnati on Sept. 8.