In nine starts for Triple-A Oklahoma City this year, Harrell was 5-2 with a 1.72 ERA. He will remain on the Major League roster for the rest of the season.
The 26-year-old right-hander pitched in three games for the White Sox earlier this year and eight games last year, including three starts. He's 1-0 with a 5.28 ERA in 11 career games.
"I've only seen him on tape, but I know he's thrown the ball extremely well," Astros manager Brad Mills said. "We're really excited about watching him pitch, and this affords us an opportunity to watch him. It's going to be kind of neat to see him."
Harrell will face no small task against Milwaukee, even though the Brewers come into the series having been swept in three games by the Cardinals.
While the Brewers are 31-38 on the road this season, they have won 10 of their last 13 away from Miller Park.
Within the National League Central, Milwaukee is 41-24 and swept Houston in three games Aug. 5-7 at Minute Maid Park.
"It's important for us to win games," Brewers manager Ron Roenicke said. "Whether it's today, whether it's the next series, it's important to win games.
"We've still got a lot of games to play. Another series against [the Cardinals]. But we can't look past Houston either. We need to go in there and play them like we did the last time."
Brewers: Greinke gets the nod
Zack Greinke (13-5), a former Cy Young Award winner, will counter Harrell. Greinke has gone 10-0 with a 2.96 ERA at home but has struggled more on the road, with a 3-5 record and a 5.56 ERA.
The 27-year-old right-hander has a 4.05 ERA in 22 starts this season, his first with the Brewers. Over 135 2/3 innings, Greinke has struck out 162.
In August, he went 5-1 with a 3.02 ERA.
The Mets on Thursday selected right-hander Adrian Rosario and left-hander Daniel Ray Herrera from the Brewers to complete the teams' mid-July trade for reliever Francisco Rodriguez.
Astros: Starters continue to shut down opponents
J.A. Happ threw seven scoreless innings Wednesday, lowering the ERA of Astros starting pitchers to 1.76 in the past seven games.
During the past 46 innings, Houston starters have allowed just 22 hits and nine earned runs. Opposing batters are hitting .170 (37-for-218) in that span.
Carlos Lee is riding a season-high 12-game hitting streak.
Reliever Fernando Rodriguez has struck out 46 batters in 42 innings.
Clint Barmes is 4-for-7 (.571) with an RBI against Greinke in his career.
In a take-off of the San Francisco Giants' popular "Orange Fridays," the Brewers want fans to "Go Gold" on Sept. 10 for a game against the Phillies at Miller Park. The Brewers will wear special gold jerseys for the game, presumably similar to the ones they sported for Cerveceros Day back in June, and fans are encouraged to wear their own gold gear in the stands.