Coleman, who was the primary backup to starting tackles Joe Staley and Mike McGlinchey, was lost for the season after he suffered a fractured fibula and dislocated ankle during the 49ers' preseason opener on August 10 against the Dallas Cowboys.
Marshall, 30, played college football at TCU and the Green Bay Packers made him a fifth-round selection in 2010. He also spent time with the Oakland Raiders, New York Giants, and Cincinnati Bengal during his professional career.
Marshall appeared in 14 games with two starts last season with the Bills and Panthers. He signed with the New Orleans Saints on May 22 but was placed on injured reserve on August 31 and released the next day.
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