Mastrobuoni was optioned to Triple-A Iowa on Monday, per MLB's transaction log.
Mastrobuoni went 0-for-10 at the dish over his last three games, and he'll be demoted to the minors now that Dansby Swanson (knee) is ready to be activated from the 10-day IL and Luis Vazquez is ready to get a shot with the big-league club. Mastrobuoni will likely be seen again in the majors this season considering his versatility.