Oswalt left Tuesday night's game against the Chicago Cubs in the second inning with lower back discomfort. He returned to Houston on Wednesday, where he was examined and given the injection by back specialist Dr. Michael McCann.
Officials hope he responds well to the injection and can make his scheduled start Sunday, but caution that it may have to be postponed depending on his progress. His condition will be monitored closely over the next few days before a decision is made.
The 31-year-old Oswalt is 6-4 with a 3.61 ERA in 22 starts this season.