Mastrobuoni cleared waivers Thursday and was sent outright to Triple-A Tacoma.
Mastrobuoni was DFA'd by Seattle after going 5-for-23 with 10 strikeouts in eight games with the big club, and he'll officially remain in the organization after passing through waivers unclaimed. The 30-year-old has posted just a .649 OPS at Triple-A this season and will likely spend the vast majority of what's left of the season in the minors.