The Mariners designated Mastrobuoni for assignment Saturday.
After having his contract selected by Seattle on July 3, Mastrobuoni made only three at-bats for the big-league club before being DFA'd on Saturday. This marks the second time the 30-year-old has been designated for assignment by Seattle this campaign. With Julio Rodriguez coming back from the 7-day injured list, the Mariners needed to make room for him on the active roster.