Walker pinch-hit in Saturday’s game against the San Diego Padres and was scheduled to start Sunday’s contest, but suffered a setback.
"He didn't feel as good as he hoped to. We've reconfigured the lineup and he's not in it," Pittsburgh manager Clint Hurdle said, via MLB.com. "We're coming to a crossroads here."
Walker is hitting .277 with 16 home runs and 50 RBI this season for Pittsburgh, which starts the week 2.5 games behind the Milwaukee Brewers in the National League Central.
Pittsburgh is awaiting the return of National League Most Valuable Player Andrew McCutchen, who has been out since Aug. 3 with an avulsion rib fracture.
- Scooby Axson