It's unclear how long he might be sidelined. Dorsey left Friday's practice with the injury. The source said Saturday that Dorsey had torn his biceps and would need an operation, speaking on condition of anonymity because the team had not announced anything about Dorsey's injury or the severity.
More tests and details are expected post-surgery, the source said. Another source, also speaking on condition of anonymity, said it would be premature to speculate that Dorsey will miss the entire 2014 season as more evaluations still must be done.
The Niners had a day off from training camp Saturday.
Losing Dorsey would be a huge blow 11 months after 2013 starting nose tackle Ian Williams went down with a season-ending ankle injury in a Week 2 loss at Seattle. Dorsey quickly emerged as a reliable replacement and stabilizing force for the defensive line.
Williams has undergone multiple surgeries and is currently on the physically unable to perform list. Mike Purcell, an undrafted free agent last year, could be an immediate option to fill the void left by Dorsey as San Francisco gears up for its preseason opener Thursday at Baltimore.
Dorsey was hurt on his 29th birthday. Fellow defensive lineman Ray McDonald played last season with a biceps tear.
In March 2013, Dorsey signed a two-year contract that could pay him up to $7 million with incentives.
Williams earned the starting nose tackle job out of training camp last year with Dorsey a versatile backup, then Williams went down with a season-ending ankle injury on a cut block by Seattle right guard J.R. Sweezy.
His teammates learned in a hurry about Dorsey's dedication, watching him emerge in a defense defined by its hard hits and work ethic behind the scenes.
That was among the first things defensive line coach Jim Tomsula talked about with Dorsey during his visit to the Bay Area.
Coach Jim Harbaugh was asked about Dorsey's contributions and growth Friday before the injury occurred during practice.
''It's been A-plus, plus,'' Harbaugh said. ''What a great thing having him last year. Fit in so seamlessly with that group of talented defensive lineman. Was a great contributor, a starter for us. Known friend and trusted agent. Just thank him for being good, thank him for being who he is and how he goes about his job. He's a team guy all the way.''
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