Oswalt left Friday's game after one inning with a strained left hip abductor, an injury that has plagued him throughout this season.
As a result, he's no longer slated to be the first pitcher back out after the break.
"I'm slotted to go fifth, at the back of the rotation, which gives me 11 days of rest," Oswalt said. "We might push it back from there. After the break, I'll throw a couple bullpen sessions and see how it feels."
Manager Cecil Cooper said it was too early to think about whether the Astros would need to call up a pitcher to complete the rotation next week.
"We'll talk about that Sunday," Cooper said. "We were anticipating [Oswalt] being the first guy out of the chute, but that probably won't happen. We'll see how he is Sunday, then make the decision."
Friday's game against the Nationals was Oswalt's first start since June 30. He threw 17 pitches, but was pinch-hit for after Cooper decided not to risk the chance of further injury. Oswalt reported that he still felt sore on Saturday when he arrived at Nationals Park.
After Oswalt's departure, the Astros' bullpen threw a combined eight innings. Cooper said he wasn't worried about their fatigue, and added that he did not plan on using Doug Brocail and Jose Valverde extensively as the break approaches. Neither threw on Friday night.
"They've thrown a lot of innings and made a lot of appearances," Cooper said. "We've relied on the bullpen all year, and that's what we'll do here."
Oswalt's injury also throws into flux the planned rotation against the Cubs next weekend, which was to be Oswalt, Wandy Rodriguez and Brian Moehler. Brandon Backe was slotted fourth, with Runelvys Hernandez tentatively scheduled to be called up in the fifth spot.
"I'm sure that [will be] jumbled up a little bit," Cooper said.
Michael Phillips is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.