Hayes' back injury is still being evaluated and there remains no timetable for his return, Noah Hiles of the Pittsburgh Post-Gazette reports.
The third baseman has been on the 10-day injured list on two occasions this season with back problems and the Pirates are still trying to get a better handle on the injury. Hayes is expected to return at some point during September, but his timeline lacks specificity. The 27-year-old is sporting a lowly .233/.283/.290 batting line this season.