Mastrobuoni is out of the lineup for Wednesday's game against the Red Sox.
Mastrobuoni had started in each of the Mariners' last four matchups versus right-handed pitching, but he'll hit the bench Wednesday while the Red Sox send southpaw Sean Newcomb to the bump. The Mariners will likely continue to deploy Mastrobuoni in a timeshare at third base with Ben Williamson until Jorge Polanco is cleared to play the field again while he manages a side injury.