Slowey, who has contributed in the rotation and out of the bullpen, is 3-6 with a 4.11 ERA in 92 innings.
The Marlins will make a corresponding move before they face the Pirates on Sunday afternoon.
Slowey has been a versatile veteran who has handled a variety of roles. Due to injuries, Slowey opened the season as the No. 2 starter, and in 14 starts, he went 1-6 with a 4.21 ERA. In six relief appearances, he is 2-0 with a 3.60 ERA.
Formerly with the Twins, Slowey made the Marlins as a non-roster invitee. He spent time on the DL in 2011 with an abdominal strain and right biceps soreness.
Due to injuries, Slowey had not pitched in the big leagues since he threw 59 1/3 innings for the Twins that year.
Slowey was loosening up in the third inning on Saturday after Tom Koehler got into early trouble.
"He was warming up down there, and his forearm tightened up on him," manager Mike Redmond said. "He wasn't able to go. Without him being our long guy, we had to make an adjustment and piece it together."