San Diego needed a quarterback in camp after Charlie Whitehurst suffered a knee injury in the team's first training camp practice.
Whitehurst signed a two-year, $3.05 million contract with the Chargers in March.
Whitehurst and the Chargers didn't disclose the exact nature of the injury but Whitehurst did say "it's not bad."
Starting quarterback Philip Rivers has started 96 consecutive games -- all 16 games in his six seasons as the Chargers' No. 1 quarterback.
Boller, 31, started one of the two games in which he played for the Raiders last season and completed 15 of 28 passes for 161 yards, three interceptions and was sacked three times in 2011.
Boller was a first-round pick from California-Berkeley in 2003 and started nine games as a rookie with the Ravens. He was a started for most of his first three seasons and started 40 total games in five seasons in Baltimore. He played for the Rams in 2009 and signed with Oakland in 2010.