The Astros sent Keuchel to Houston after he threw on Sunday in Arlington and didn't feel well.
"This is the time of year where there's always soreness. He's really good at pitching through a lot of the irregularities that happen throughout the season, and this is just something he couldn't do," Hinch said. "When he played catch on Sunday and felt uncomfortable, we decided to send him back."
Hinch said that Keuchel has been dealing with the pain since the day after his most recent start, on Aug. 27 against the Rays in Houston. He added that Keuchel won't even play catch until he's feeling better, so there's no timetable for a return.
"We'll have to get back and see how all these tests come back and how he continues to feel," he said. "We tried to give him some gaps in time in giving him some rest periods. That hasn't solved it. He's going to see the doctors he needs to see, and until he's pain-free, he's not throwing.
"Where we're at on the calendar, it's going to bring the obvious questions, but we just don't know right now."
Keuchel (9-12, 4.55 ERA) has quietly put together a strong second half, going 6-3 with a 3.47 ERA in 12 starts since June 18, including 10 quality starts. He's thrown 168 innings this season after last year's career-high workload of 246 innings, including the postseason.
The 2015 Cy Young winner was originally scheduled to pitch against the Rangers last Friday, but he was pushed back to Tuesday with what the team was calling "fatigue" at the time.
Losing Keuchel is a huge blow to the Astros, who are already without right-hander Lance McCullers. McCullers is playing catch in Houston while he recovers from a mild sprain in his right elbow that could cost him the rest of the season.
"It's a difficult blow in that our rotation is going to need to have guys step up and pitch well and keep us in games," Hinch said. "It's never easy, and I think every team this time of year, regardless of whether you're in it or not, probably has some circumstances they've had to overcome. This is going to be ours.
"We have a couple of starting pitchers that were in our rotation to start the year, or in our plans [that are injured] ... . Someone's going to step up. The 'next man' mentality has to be what we operate with. We'll see how we address the rotation moving forward. Peacock gets the opportunity tomorrow to pitch against the Indians, but we'll have to make do with what we have."
Peacock went 5-6 with a 4.23 ERA in 22 games (21 starts) for Triple-A Fresno this year. He appeared in five games in August in relief for the Astros, allowing three runs and four hits in seven innings.