The right-hander, who suffered a strained left hip abductor on June 30 against the Dodgers, was expected to throw long tosses on flat ground Friday before taking a bullpen session on Saturday.
Both Oswalt and manager Cecil Cooper said nothing would be known until after the bullpen session, but while Cooper said he hoped Oswalt would be close to 100 percent, the ace was not so sure.
"[I'm] still sore, kind of, actually," he said. "It's 50-50. Maybe 51 [percent] not good."
Meanwhile, the Astros plan to recall Runelvys Hernandez for Monday's start against Pittsburgh. Hernandez was optioned to Triple-A Round Rock over the All-Star break to get two more starts to stay in line to pitch in Houston again.
While Houston has a couple of options for Tuesday if Oswalt cannot pitch, Cooper said previous starter Chris Sampson is not one of them.
The current reliever made one start just before the break, allowing one run on two hits in five innings of work in a spot start for Oswalt on July 5.
"He's pitched extremely well out of the 'pen," Cooper said. "We tinkered with it once, but I don't think we should continue. We need to keep him in a spot where I think he can settle into."
Instead, general manager Ed Wade said Jack Cassel could be called up from Round Rock to start on Tuesday.
Cooper said he hopes his top guy will be ready, but does not want to rush anything.
"It is difficult," he said. "You've got to hope the player will be honest with you and tell you just how he feels. And you would like to have him in there because he's our No. 1 guy. But if anyone doubts that he's not 100 percent, you've got to err on the side of being a little cautious."
Krysten Oliphant is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.