Fernando Nieve replaced Oswalt, and the start of the game was delayed 10 minutes in order to give the emergency starter time to warm up.
Taylor Buchholz will start on Monday against the Cubs, instead of Nieve, and Wandy Rodriguez will move up a day to start Tuesday. Because of last week's off-day, the starters still will be able to pitch on full rest.
If he recovers from his back issue in the next day or two, Oswalt could start Wednesday. But given his current condition, that scenario appears unlikely.
It's too early to determine if Oswalt will have an MRI or if his injury will force him to the disabled list. He'll first be evaluated by team doctors, and then decisions will be made.
"I don't know if it's a spasm or a strain," manager Phil Garner said. "We'll let the doctors try to assess that in the next day or two."
In his previous outing -- last Monday in St. Louis -- Oswalt left after six innings with a tight right hamstring. He was given a clean bill of health a few days later, but he hasn't ruled out a correlation between that injury and the back problem that popped up Sunday.
"I may have changed my mechanics a little bit in St. Louis," Oswalt said. "But it seemed like I would have felt it [in my back] in the next day or two. I really didn't feel it until the fourth day [after the start]. I don't know."
Oswalt, who has never had a back injury in his career, admitted that had the team been playing better, he probably would have opted to err on the side of caution and not start Sunday. He also might be unconsciously changing his approach on the mound, in an attempt to overcompensate as he tries to help the struggling Astros -- now three games under .500.
"I'm trying to do too much," he said. "The team is struggling. I feel like I need to do more than I can do. Under normal circumstances, I would have told them, 'Let's give it a few more days and see how I feel.' But since the team was struggling, I was going to try to [push it]."
Until Oswalt experienced this pain -- which has settled in the left side of the middle of his back -- he never thought about how much a pitcher relies on that area of his body.
"I didn't realize you use your back that much," he said. "You use it for everything. I've got to get it well and go from there. I wish I could pitch with it hurt, but there's no way."