St. Louis ? 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 rookie right-hander Yusmeiro Petit from Triple-A Tucson to pitch in Johnson’s place.

The 43-year-old left-hander said he didn’t believe the injury was career-threatening.

“That hasn’t even been part of the conversation,” Johnson said. “I think when you’ve been around as long as I have, you manage whatever comes across. It’s just one more obstacle.”

Johnson is 4-3 with a 3.81 earned-run average 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.

The plan calls for 10 days of rest and treatment, after which the injury will be re-evaluated. Johnson has taken two cortisone injections this year to relieve symptoms, including one injection last week, but said the second shot was not as effective as the first.