Gradkowski will undergo surgery for the injury on Friday but is expected to be fully healed by the start of training camp in July, his agent, Ralph Cindrich, confirmed in an e-mail. The injury was first reported by ProFootballTalk.com.
Russell lost his job midway through last season because of ineffectiveness and is expected to compete with Gradkowski to get it back this season.
The Raiders announced the signing of Boller shortly after coach Tom Cable addressed the media about the upcoming draft and before reports of Gradkowski's injury surfaced.
Boller never lived up to his billing after being drafted 19th overall by Baltimore in 2003. He started nine games as a rookie and all 16 the following season. But because of injuries and ineffectiveness, he eventually lost his job and played last season in St. Louis.
Boller is familiar with new Raiders offensive coordinator Hue Jackson, who was the quarterbacks coach in Baltimore in Boller's final season with the Ravens. He did not play at all in 2008 because of a shoulder injury.
The former Cal star has made 46 starts in his career, throwing 50 interceptions and only 48 touchdown passes. He has completed 56.8 percent of his passes and has a career passer rating of 70.6.
The Raiders are looking for better play out of their quarterbacks this season and were linked to trade talks with Philadelphia for Donovan McNabb before he was dealt to Washington.
Russell had a disappointing third year in the NFL. He completed 48.8 percent of his passes, with three touchdowns, 11 interceptions and a 50.0 passer rating that was the lowest in the NFL in 11 years before losing his job to Gradkowski.
Russell has been participating in offseason workouts, but Cable would not get into details about how he has looked.
"He's here. He's working," Cable said. "He's doing everything that everybody else is doing, has been here since the start of the second week. Has been doing it like everybody else. That's all I can tell you."
Gradkowski provided an immediate spark after replacing Russell, leading Oakland to comeback victories in two of his first three starts. He got hurt midway through his fourth start, suffering a torn medial collateral ligament in his left knee and a partially torn MCL in his right knee, and did not play the rest of the season. He completed 54.7 percent of his passes with six touchdowns and three interceptions on the season.
Frye started the final three games last season, completing 60.9 percent of his passes with one touchdown and four interceptions.