Mastroianni hit .252 in 77 games with the Twins last season, but has only appeared in eight games in the 2013 season. The 27-year-old has been in Fort Myers since July 7 and will make his first Minor League rehab start on Saturday.
"That worked kind of well because he'll probably [be the designated hitter] and play five innings tomorrow with Sunday off. Then he'll be back on Monday," assistant general manager Rob Antony said.
Ramirez will also be in Fort Myers for his rehab assignment, starting on Monday. He has hit .244 in 23 games for the Twins this year.
Jacob Thorpe is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.