Hurdle said the injury is something that has bothered Hart for quite some time, but not one instance caused it.
The 33-year-old right-handed hitter, acquired this offseason, has struggled to find a place in the Pirates' everyday lineup. Hart, hitting .222 with a .246 on-base percentage, has started just 11 games for Pittsburgh this year.
"After yesterday talking with him, we feel that this is the proper time to get him some rest, to get it right, and then get him some at-bats, and see where we can take it," Hurdle said.
Meanwhile, Hurdle said the batter-for-pitcher decision resulted from the heavy workload his bullpen has endured recently. Over just the last two games, all seven of Hurdle's relievers have pitched, combining for 12 1/3 innings of work.
Vance Worley, who threw 4 1/3 innings on Sunday, and Arquimedes Caminero were likely unavailable for Wednesday's game, so Volstad could potentially get some early innings.
"We felt like this was the best move we could make to protect all the guys out there [in the bullpen]," Hurdle said.
John McGonigal is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.