Locke, one of five players the Pirates called up Saturday in a first wave of reinforcements, will start Monday in the first game of a three-game series against the Astros at PNC Park.
Houston's starter will be Edgar Gonzalez, who will be recalled from Triple-A Oklahoma City, where he was 1-0 with a 0.69 ERA in two starts for the RedHawks after pitching for Monterrey in the Mexican League.
Locke, who was 10-5 with a 2.48 ERA in 24 starts at Triple-A Indianapolis, takes the spot in the starting rotation previously occupied by Erik Bedard with the Pirates, who are in the thick of the National League Wild Card chase.
They are in fourth place in the Wild Card standings, 1 1/2 games behind the Cardinals for the second spot and a game behind the Dodgers.
The Pirates have been anticipating Locke's move to the starting rotation, and they expect him to stay there for the rest of the season and beyond.
"I'm not under the assumption it's a one-shot deal," manager Clint Hurdle said.
Locke made four starts as a callup last September and appeared in two games in relief for the Pirates earlier this season, and says he ready for this opportunity.
"Things were going good in Triple-A, and I'll try to carry over some of that confidence here, and attack them," Locke said Sunday. "I'm just excited. It's nice to be here. My confidence is through the roof. It won't be any easier [than last September], but it'll be a lot more comfortable."
There is a bit of a buzz within the Pirates' organization regarding Locke's start after he was named pitcher of the year at Triple-A Indianapolis, Hurdle said.
"Our development people really look forward to seeing him at this level, based on what did all season," Hurdle said. "He's made a lot of progress -- with his ability to locate, to mix speeds, with the the command -- all of it. The next step is to check it out here. We want to get some looks at him."
The Astros called up outfielder Jordan Schafer, right-handed reliever Jose Valdez and utility player Matt Downs from Oklahoma City and expect to add others, depending on when the season ends for the RedHawks, who are in contention for the Pacific Coast League championship.
"In the next day or two, we'll find out where we stand with bringing some more players up to help," interim Astros manager Tony DeFrancesco said.
Astros: Greene helping out
Shortstop Tyler Greene hit his fifth home run in 20 games since joining the Astros in Sunday's 5-3 loss to the Reds. Greene had four home runs and hit .218 in 77 games for the Cardinals, and is hitting .254 with the Astros.
Pirates: McCutchen power turned down
All-Star center fielder Andrew McCutchen was 1-for-5 in Sunday's 12-8 loss to the Brewers, and is hitting .344 with 24 home runs and 80 RBIs. McCutchen's power numbers have declined recently, as he has hit just two home runs since Aug. 1.
The Astros make their NL Central farewell tour in the month of September with five road series against division rivals Pittsburgh (Monday through Wednesday), Cincinnati (Friday through Sunday), St. Louis (Sept. 18-20), Milwaukee (Sept. 28-30) and the Cubs (Oct. 1-3).
It will be the final NL road series in each NL Central city for the Astros, who are moving to the AL next year.