Newberry's agent, Doug Hendrickson, told The Associated Press on Tuesday the former longtime starting center for the San Francisco 49ers hurt his left knee.
Hendrickson said Newberry's knee "just kind of went out on him recently when he was pushing hard to get ready for training camp."
The 33-year-old Newberry was expected to provide depth at guard and center.
Newberry started three games for San Diego last season after starting each of his 14 games with Oakland in 2007.
Newberry's retirement was first reported by the Atlanta Journal-Constitution.