To make room for Foltynewicz on the roster, third baseman Chris Johnson will be placed on the 15-day disabled list with a broken left hand.
Foltynewicz, a top prospect acquired in the January trade that sent slugger Evan Gattis to Houston, went 0-3 with a 2.08 ERA in four starts at Triple-A Gwinnett this month. He struck out 30 in 21 2-3 innings.
In 16 relief appearances with the Astros last year, Foltynewicz went 0-1 with a 5.30 ERA. Houston drafted him in the first round of 2010.
Johnson was injured while sliding into second base in the fourth inning of Thursday's 5-1 loss to Cincinnati.