CHICAGO -- Rookie Jordan Lyles spent Monday morning getting used to being in the big leagues, ready to make his Major League debut on Tuesday night at Wrigley Field against the Chicago Cubs.

If things go well during this callup from Triple-A Oklahoma City, his stay could be a lot longer than the two starts he's slated to get in the absence of Wandy Rodriguez, who was placed on the disabled list Saturday with swelling in his left elbow.

"When he came in today I said, 'You could be here for two starts or 20 years,'" Astros manager Brad Mills said. "He's a pretty good pitcher. We just want him to continue to be that pitcher. That's all. He doesn't have to try to do any more or any less or anything else -- just be himself and pitch, and he'll be fine."

Lyles, a 20-year-old right-hander, nearly made the club out of Spring Training and has impressed at Oklahoma City. He was 3-3 with a 3.20 ERA and performed even better in his last seven starts (3-1, 2.00 ERA). He admitted there are some nerves building for his Tuesday debut with the Astros, but said that won't likely change his focus or approach.

"There's going to be nerves there, but it's more just being anxious to get out there," Lyles said. "Being nervous is not a bad thing. I just want to make my pitches. That's really all I'm concerned about. If I'm here for the long run, that will take care of itself. If not, I'm just going to continue coming to the field with the mindset of getting better every day."

That kind of attitude is the reason Mills doesn't expect nerves to be an issue when Lyles takes the mound to square off against Cubs starter Carlos Zambrano.

"He's so level-headed that [his pulse] is almost like a flat line, because that's just how he is and that's fine," Mills said. "His demeanor has always been very confident, and I don't expect to see that change at all with him."

Lyles was pulled a little early from his most recent start after throwing just 77 pitches, but Mills said that was precautionary and the rookie won't be on a set pitch count.

"It hasn't sunk in yet at all," Lyles said. "I'm just getting accustomed to everything and I'm ready to get out there on the field. It's going to be special."

Happ finding his groove on the hill for Astros

CHICAGO -- After a rough start to the season, J.A. Happ has strung together seven solid starts in a row to lower his ERA and give Houston a chance to win games -- something the Astros need to get back in the National League Central race.

His ERA is still 4.66, but it's just 3.35 in the last seven starts, which includes a two-hit, one-run showing over six innings in Sunday's 4-2 loss to the D-backs.

"Confidence is everything in this game," Happ said. "If you can maintain that confidence and just go out there and be consistent with it, people can get on some pretty impressive rolls. That's how guys get hot. They have the confidence. It's not that they lack ability. I'm trying to get that back, and you get confidence by having success. It's kind of a relationship in that way."

Astros manager Brad Mills certainly likes what he's seen recently from the 28-year left-hander and expects it to continue -- not only this season, but also into the future.

"He's going to continue to get better," Mills said. "I think you can look at his career as maybe a microcosm of this season. He's going to continue to get better -- not only this year, but on into the years to come -- the way he throws the ball."

Astros trying to build confidence

CHICAGO -- Almost as much as wins, the Astros are seeking any measure of confidence they can find while sitting in last place in the National League Central.

Having dropped four of their last five games, the Astros are hoping to turn it around in Chicago as they begin a seven-game road trip by focusing on small victories within games that manager Brad Mills thinks will lead to wins in the standings.

"We're going to go after it each day and each game and each inning and each at-bat and each pitch to win those categories," Mills said. "You win an inning and you win the at-bat and start putting those things together. We want to gain that experience and that knowledge of how to do those things to win ballgames.

"Confidence is a big thing with every ballclub, and how you approach it and how you go about it is huge. If you can gain that confidence in those little things, it will continue to build as we move forward."

Worth noting

• Hunter Pence's RBI double in the first inning on Monday extended his hitting streak to 11 games. At 10 games, Pence had the third-longest active streak in the National League starting the day. It was also his eighth RBI in the last 10 games.

• The Astros improved their batting average with runners in scoring position in the first inning on Monday against the Cubs, when Jeff Keppinger doubled home Pence for a 2-0 lead. Starting the day, Houston was second in the NL and fourth in the Majors in that category with a .279 average.

• J.A. Happ, who's originally from Peru, Ill., and played at Northwestern, was originally scheduled to start a game in this series against the Cubs. The Astros' rotation got switched around, however, and Happ instead started on Sunday at home against Arizona. He still, however, had a number of ticket requests from family and friends at Wrigley Field.

• Astros left fielder Carlos Lee is third among active players with 35 home runs against the Cubs and also came into Monday's game with the third-highest batting average on Memorial Day among active players (.435, 10-for-23).

• Before Monday's game, the Cubs had a trained bald eagle circle above the field before landing on its trainer's arm to commemorate Memorial Day.