The 27-year-old Gray is 11-8 with a 3.84 ERA this season. The right-hander was scheduled to start the series finale against the Arizona Diamondbacks on Wednesday. He was coming off eight scoreless innings of five-hit ball in his last start on Aug. 16 against Miami.
Gray made 25 starts and struck out 150 batters in 150 innings this season.
The Rockies called up Tim Melville from Triple-A Albuquerque to make the start Wednesday in Arizona.