The Suns are expected to release Mitchell to create an additional $2.2 million in salary cap space, giving the team a total of $6.2 million available and an open roster spot.
Phoenix signed Tolliver to a two-year, $6 million contract in the offseason, with the idea it would help make up for the loss of Channing Frye. But Tolliver played sparingly and was not part of the regular rotation as the season went on.
He played in 24 games for the Suns, averaging 3.3 points per game.
Mitchell, a second-round draft pick of Detroit last season, was playing for Grand Rapids of the NBA Developmental League.