Harrell is starting in place of Brett Myers, who is away from the club. His wife, Kim, gave birth to their fourth child Thursday afternoon.
The right-handed Harrell will make his first start for Houston, which claimed him off waivers from the White Sox in July. He made three relief appearances for Chicago this season, giving up four runs in five innings.
Harrell, 26, has made nine starts for Oklahoma City, going 5-2 with a 1.72 earned run average.
Also on Friday, the Astros activated reliever Enerio Del Rosario from the 15- day disabled list. He had been out since August 6 because of a right shoulder strain. In 44 games this season, Del Rosario has posted a 4.50 earned run average, with 26 strikeouts and 25 walks in 46 innings.
All have played for the Astros this season. Johnson and Wallace both started the season in the majors, but have been in the minors since the beginning of August.
Johnson hit .245 with a .286 on-base percentage and six homers in 92 games for the Astros, while Wallace batted .268 with a .345 OBP in 101 games. However, the first baseman slugged just .375.