Mastroianni (oblique) was activated from the DL on Monday, but the Twins outrighted him off the 40-man roster, Rhett Bollinger of MLB.com reports.
Mastroianni only fit in a brief showing with the Twins this season before landing on the disabled list in late May. When he attempted to start his rehab assignment in mid-June, he experienced a setback, pushing back his first rehab appearance to July 1. In three games with Triple-A Rochester in the meantime, the outfielder tallied at least one hit on each occasion, but the organization will instead turn in another direction with his 40-man roster spot.