Oswalt, if healthy, would have started Sunday at St. Louis. The Astros called up right-hander Bud Norris from Triple-A to replace him, and Norris beat the Cardinals, 2-0, in his Major League debut.
Oswalt's next start was first pushed back to Tuesday and then Saturday. General manager Ed Wade said it was still possible that Oswalt could be placed on the disabled list if he is not ready to go Saturday vs. Milwaukee. Wade said that if Oswalt is put on the DL, it could be backdated 10 days to his last start.
The Astros are expected to recall right-hander Felipe Paulino from Triple-A Round Rock to start Tuesday night's game.
Oswalt said he will go back to his doctor Tuesday and throw another bullpen session Wednesday.
"He was not as good as all of us anticipated," manager Cecil Cooper said of Oswalt's Monday bullpen session. "We'll try again Wednesday.
"He's our ace, he's our No. 1. We need Roy to be healthy and 100 percent and pitching the way Roy pitches. Without him makes it very difficult. I think he wants to get out there and we'll have to hold him back a little bit. When you're a competitor, you want to go."
The rotation is even a little more dicey with Wandy Rodriguez, the team's best left-handed starter, questionable because he strained his right hamstring running the bases in his last start Saturday at St. Louis.
Right-hander Brian Moehler is scheduled to start Wednesday and the Astros are off Thursday. Norris is the likely starter Friday when the Astros open a three-game series at home against Milwaukee.
"It's flip a coin after that," Cooper said. "Saturday it could be back to [Monday's starter Mike Hampton] and Sunday could be Paulino. We have to protect these two guys [Oswalt and Rodriguez]. That's our one and our two."
Reserve outfielder Darin Erstad, the team's No. 1 pinch-hitter, ran sprints before Monday's game. He has been on the disabled list since July 19 with a strained left hamstring.
Erstad, 35, will go out for a two-game rehab assignment with Double-A Corpus Christi on Tuesday and Wednesday, then be re-evaluated.
First baseman Lance Berkman, on the DL with a strained left calf muscle, could be activated as soon as Friday.
"I feel good," Berkman said. "It hasn't affected my batting at all. It's just when I run."
Cooper felt no reason to give his team a pep talk, despite the rash of injuries.
"With a veteran club I don't think you have to say a whole lot," he said. "We have good, veteran leadership. We're not down. We're just having a difficult time right now [losing seven of nine], because of the injuries mainly."
Wade tried to be optimistic.
"With a third of the season left to play, we have a lot of time to get these injuries healed up," he said. "None of them are season-ending type things. We can be patient and work through a little rough spot now knowing we're going to have a chance to have these guys for the lion's share of the stretch drive."