The left-hander was originally pushed back from Friday's start and then scratched from Tuesday's start against the Cleveland Indians.
On Monday, the reigning American League Cy Young Award winner flew back to Houston for further evaluation on the shoulder. Manager A.J. Hinch told reporters he didn't have a feel for whether Keuchel will return this season.
"We'll have to get back and see how all these tests come back and how he continues to feel," Hinch said. "We tried to give him some gaps in time and give him some rest period, but that hasn't solved it. He's going to go see the doctors he needs to see, and until he's pain free, he's not throwing.
"Where we are in the calendar is going to bring the obvious questions, but we just don't know right now."
The Astros recalled right-hander Brad Peacock from Triple-A Fresno to make Tuesday's start against the Indians. Peacock had a 3.86 ERA in five relief appearances with Houston earlier this season, and went 5-6 with a 4.23 ERA at Fresno.
Keuchel has been unable to follow up last season's career-best campaign when he went 20-8 with a 2.48 ERA. He got off to a slow start this year and a strong August boosted his season stats to 9-12 and 4.55.