Mastroianni begins his rehab assignment with Triple-A Rochester on Friday, Rhett Bollinger of MLB.com reports
Mastroianni has been hampered by an oblique injury that's kept him out of games since May 21. The outfielder began running sprints and playing catch several weeks ago so he should be nearing 100 percent at this point. The 30-year-old will likely participate in a few tune up games with Minnesota's Triple-A affiliate and could return to the majors early next week.