Mastrobuoni cleared waivers Tuesday and was outrighted to Triple-A Tacoma.
Mastrobuoni was dropped from the active and 40-man rosters Saturday to make room for Julio Rodriguez's (leg) return from the IL, but the former will officially remain in the Mariners' system after passing through waivers unclaimed. Mastrobuoni, 30, has slashed just .172/.310/.293 through 71 plate appearances at Tacoma and is likely to spend the vast majority of what's left in the season at Triple-A.