Walker (back) underwent an MRI on Wednesday which showed no structural damage, though his availability for Sunday's scheduled start in Philadelphia remains up in the air, Tim Healey of Newsday reports.
The veteran right-hander was pulled from Tuesday's start after two innings due to back spasms, but the injury appears to be a minor concern. However, it remains to be seen if Walker will be able to make his next scheduled turn through the rotation or if he'll need to be pushed back a few days.