Fuld is not due to join Minnesota until its game Tuesday at Tampa Bay, leaving the club one player short for its series finale against the Royals on Sunday.
Fuld was hitting .200 with a homer and four RBIs in seven games for the Athletics. He had spent parts of the previous three seasons with Tampa Bay after breaking in with the Chicago Cubs.
The A's designated Fuld for assignment April 12.
Mastroianni has been with the Twins for parts of the past three seasons. He was 0 for 11 in 12 plate appearances over seven games this year.