Boone has held out since early in the offseason. The two sides are not close to a contract extension and Boone is believed to be prepared to stay away from the team until he gets a new deal. He has two seasons remaining on his contract that will pay him about a total of $4 million.
Boone signed a five-year deal in 2011 when he was a backup and the contract was essentially for backup money. He has developed into one of the better guards in the NFL.
Among the teams that could use a guard are the Buccaneers and Giants. Boone can also play tackle.
Trading Boone, a key part of the 49ers' offense, will likely be a last resort for the 49ers.
Joe Looney is working with the first unit in Boone's place. He struggled in the preseason opener at Baltimore on Thursday.