Arizona pitcher Randy Johnson was put on the 15-day disabled list for the third time this season, sidelined Tuesday because of a sore back right before his scheduled start at St. Louis. The Diamondbacks promoted 22-year-old righty Yusmeiro Petit from Triple-A Tucson to pitch in Johnson's place. Johnson is 4-3 with a 3.81 ERA this year. The 43-year-old lefty had back surgery last October and recently was diagnosed with a herniated disk. Johnson began the season on the disabled list, then returned to the DL on June 11. The latest move was made retroactive to June 29.