The Cubs optioned Mastrobuoni to Triple-A Iowa on Tuesday, Meghan Montemurro of the Chicago Tribune reports.
With Isaac Paredes now active and set to take over as Chicago's primary third baseman, Mastrobuoni no longer has a place in the Cubs' infield unit and will now head back to Triple-A. The 28-year-old slashed .260/.283/.300 in 53 plate appearances after being recalled in mid-June.