O'Brien (ribs) was designated for assignment Thursday, Jordan McPherson of the Miami Herald reports.
O'Brien has been sidelined for nearly a month with a rib injury. Prior to landing on the shelf, the 28-year-old slashed just .167/.255/.262 in 14 games, so the Marlins decided to drop him from their 40-man roster rather than wait for him to get healthy. This move frees up a spot on Miami's 40-man roster for Thursday's starter, Zac Gallen, whose contract was purchased from Triple-A New Orleans.