Volstad lasted only five innings and gave up four runs, three earned, against the New York Mets on Friday. That lifted his ERA to 5.58, third-highest in the major leagues.
Volstad is 5-8 in 20 starts. Manager Jack McKeon said he anticipated Volstad would rejoin the rotation later this season.
The 24-year-old Volstad was also demoted to New Orleans in July last year. He returned to the Marlins and went 8-1 in his final 13 starts.