The club revealed the diagnosis today, one day after Johnson visited with Dr. Robert Watkins in Los Angeles. Johnson went on the 15-day disabled list June 11 with a tight glute.

"He does have a bulge in there, a herniation," manager Bob Melvin said before the Diamondbacks played Tampa Bay today. "Obviously not the news we want to hear, yet he thinks he is healthy enough to go ahead and prepare for his next start. When that is, we're obviously not sure."

The 43-year-old left-hander had back surgery last October. The Diamondbacks acquired him in a trade with the New York Yankees in January, and he opened the season on the disabled list.

Johnson played catch in the outfield and was scheduled to speak to reporters after batting practice.

News of Johnson's latest back problem comes after he had won four consecutive decisions for the upstart Diamondbacks.

Johnson is 4-2 with a 3.52 ERA in nine starts, and he has 70 strikeouts and 11 walks in 53 2-3 innings. Melvin said it's unclear when Johnson's latest injury occurred.

"He's been pitching pretty well, and there's a good chance that he's been pitching some with this herniation in there," Melvin said. "He's been a significant factor. Every time he takes the mound, he's a psychological advantage for us whether he pitches well or not."

Melvin said Johnson seemed hopeful that he will be able to rejoin the team when he's eligible to come off the disabled list June 26. But Melvin said the team will wait until it sees how Johnson responds to throwing on the side the next two days.

"How he feels after that will dictate how we go about setting up his schedule," Melvin said. "It's probably a stretch to get him ready for the 26th right now."