Surgery will not be necessary, but Casey will need to rest and could be sidelined for six-to-eight weeks.
The Pirates said Casey fractured the transverse process in his lower back. The transverse process is where the muscles attach to the vertebrae. Casey was injured in his first game back after missing three starts with a bruised rib.
He suffered the injury while stretching to catch a throw from Pittsburgh second baseman Jose Castillo. As John Mabry of the Chicago Cubs was running out an infield single in the third inning, he stumbled crossing the bag and clipped Casey with his hip or left elbow.
In eight games this season, his first with the Pirates, Casey is hitting .313 with two homers and five runs batted in. Pittsburgh acquired Casey from Cincinnati in an offseason trade for pitcher Dave Williams.
With Casey out, first baseman-outfielder Craig Wilson is expected to get most of the playing time. A year ago, it was Wilson that missed extensive playing time -- about 100 games -- with two left hand injuries a year after he led the team by hitting 29 home runs.
"You never want to see someone get hurt like this," Wilson said. "That's definitely the last thing you want. One thing is it's really boring when you're not playing."
The Pirates filled Casey's roster spot by purchasing the contract of infielder/outfielder Mike Edwards from Triple-A Indianapolis. Edwards was hitting .316 with a home run and three runs batted for the Indians this season.
"As a general manager, you have to be prepared because these things can happen and do happen to every team every year," general manager Dave Littlefield said. "But we're pleased with depth we created in the offseason, and this will give other people some opportunities."
To make room for Edwards on the 40-man roster, the Pirates transferred pitcher Kip Wells from the 15-day to the 60-day disabled list. Wells is expected to be sidelined until the All-Star break after undergoing surgery to correct a blockage to an artery in his right arm.