Brewers manager Ken Macha said Wednesday that Parra, who is 3-8 with a 7.52 ERA in 13 starts this season, will start against St. Louis in the series finale.
Parra was demoted immediately after a 7-1 loss to the Chicago White Sox on June 13, after going 0-4 with a 13.50 ERA in his previous five starts.
Parra was 1-2 with a 2.92 ERA in four starts with Triple-A Nashville.
Macha said Dave Bush will have a rehab start at Single-A Wisconsin on Friday.
The right-hander, who is 3-4 with a 5.67 ERA this season, has been on the DL since June 23 with a small tear in his right triceps.