But a couple of hours before game time at Busch Stadium, the Astros announced that the left-hander, who twisted his left ankle while shagging fly balls in Milwaukee on Saturday, was scratched from the opener with the Cardinals. Right-hander Taylor Buchholz, who was recalled from Triple-A Round Rock earlier in the day, started in Pettitte's place.
Confident that this ailment is just a temporary setback, Pettitte said he was 100-percent sure that he would be starting on Tuesday. In fact, if the Astros hadn't needed a spot starter on Tuesday, Pettitte probably would have tried to pitch through the pain. But Buchholz was fully rested after making his last start for Round Rock last Wednesday, making it easy to take the precautionary measure with Pettitte and give him an extra day to rest.
"I'd be pitching if we didn't need a spot starter tomorrow," Pettitte said. "It made it easy to push me back and say, 'Let Taylor pitch tonight, and I can go tomorrow.'"
Pettitte was running around the outfield during batting practice at Miller Park on Saturday when he "rolled" his left ankle. He still had hope in the hours leading up to Monday's game that the mishap would not prevent him from staying on schedule.
Manager Phil Garner doesn't appear to be concerned about any lingering effects.
"I don't think the ankle's an issue," he said. "I really don't."
"He can probably go today, but it's probably best to give him an extra day," general manager Tim Purpura said. "From a pitching point of view, he seems to be fine. But it's also covering first, fielding bunts and running the bases."
Pettitte's only regret is that his new schedule will prevent him from pitching on full rest on the last day of the season. Had he pitched on Monday, he would have been in line to start on Oct. 1 in Atlanta on full rest. Now, if he pitches that day, it'll be after only three days off.
"That's what I lose," he said. "We kind of weighed it, and I said, 'If we're in a position to win that last game of the year and get to the playoffs, I can try to come back on three days' rest and go from there.' Hopefully, we're in that position."
Back in the mix: Buchholz last appeared in a big-league uniform on July 29, the day he was optioned to the Minor Leagues after struggling in several of his 18 starts for Houston.
Although Buchholz looked utterly dominating in a couple of outings -- he threw 8 2/3 scoreless frames against the Pirates on April 22 and a complete-game shutout against the Rangers on June 21 -- he also had some forgettable starts. In his last three trips to the mound, he allowed 16 earned runs over a combined 12 innings, and that was enough to earn him a ticket to Round Rock.
While in the Minors, Buchholz worked to first and foremost regain his confidence.
"I think I was pitching away from contact after getting hit around here a little," he said. "I have to attack the zone, get ahead of the batters. I wasn't doing that in my last four or five starts after the All-Star break. I got down there and it took about three or four starts to get back to where I was at the beginning of the year. I got my confidence back."
Clemens on Friday: Roger Clemens left a message with Purpura on Monday, reassuring the GM that he would be ready to start when the Astros return home on Friday.
Clemens has not pitched since he made an early exit from his Sept. 4 start in Philadelphia. He left with a right groin strain and has been receiving treatments for the pain while the team continued its nine-game road trip.
"He said it's going well, and he's targeting Friday," Purpura said.
Milestones: Orlando Palmeiro knew that he was nearing the 100 mark for pinch-hits, but he had no idea that his milestone-reaching double on Sunday would put him into such an exlusive category.
Palmeiro is the 18th player all-time with at least 100 career pinch-hits and the 10th to achieve the feat since 1974. When Palmeiro -- a veteran of 12 big-league seasons -- heard that, he acknowledged that the milestone was a pretty significant marker in his career.
"I knew I was getting close to 100," he said. "I didn't know it was that big of a deal. But a guy with my talent level, to be able to feel like I'm in a small group, it feels good."
Anniversary: As the nation acknowledged the five-year anniversary of the tragic events of Sept. 11, 2001, on Monday, Pettitte recalled a few memories of his own from that terrible day.
Pettitte, a New York Yankee at the time, remembers how much the fans rallied around the Yankees and looked to them to divert their attention, however temporarily, from the horrific realities that surrounded the city.
"It seemed to give them hope that everything was going to be OK, because the Yankees were still playing," Pettitte remembered.
Pettitte also remembered how upset he was when he didn't perform as well as he would have liked in the World Series that year, wanting so much to help bring the city a championship.
"That was the one thing that was so devastating about it," he said. "I pitched very poorly in the World Series that year, and I wanted it more for the city and the fans more than ever that year."
Coming up: The Astros and Cardinals will meet again at Busch Stadium on Tuesday at 7:10 p.m. CT. Pettitte (13-13, 4.34 ERA) will face Cardinals righty Jeff Weaver (3-4, 5.19 ERA).
Alyson Footer is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.