Pirates’ offense runs stale again against Cardinals rookie Mathews

ST. LOUIS — Don Kelly knew his team needed to find a balance. In San Diego, the Pirates took two of three from the Padres, a clear positive. The series ended in frustration, though, with Pittsburgh going 1-for-13 with runners in scoring position in a shutout loss Wednesday. “We’ve got to keep going,” the Pirates’ manager said prior to Friday’s game against the Cardinals at Busch Stadium. “[The] team has fought all year really hard, and this is no different.” Pittsburgh answered its frustration with another step backward. Facing rookie left-hander Quinn Mathews in his fourth career start, the Pirates…