The 43-year-old Johnson seemed upbeat after the club revealed the diagnosis Tuesday, 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.
"I won't back away," said Johnson, who also had a back operation 11 years ago. "I'm eager to still pitch. I've tackled this twice before."
Johnson said the disk is the same one Watkins repaired in an operation last October. Johnson said the injury can be treated with rest and medication, but it can be unpredictable.
Johnson said Watkins cleared him to resume pitching but added, "Your body's going to tell you how much and when you can."
Johnson said he felt good after throwing in the outfield Tuesday afternoon. The Diamondbacks wouldn't commit to a date for his return until they see how he responds to another throwing session Wednesday.
"Obviously not the news we want to hear, yet he thinks he is healthy enough to go ahead and prepare for his next start," manager Bob Melvin said before the Diamondbacks met Tampa Bay on Tuesday night. "When that is, we're obviously not sure."
Johnson said he had felt pain in his legs after most of his starts, and it turned out to be a symptom of the herniated disk. He didn't know what caused the latest injury.
"It has nothing to do with my age," Johnson said. "It's been injured once, and it maybe became vulnerable when I slid at third base one day. Or maybe when I was taking BP trying to hit balls off the batter's eye. Who knows?"
Three months after his operation, the Diamondbacks acquired Johnson in a trade with the New York Yankees in January. He opened the season on the disabled list.
Johnson said he asked Watkins if he had come back too soon.
"His timeframe was coming back in May," Johnson said. "Obviously, I came back in mid-April. But he didn't think that was the case because my body was ready to do all the things I was doing."
News of Johnson's setback comes after he had won four consecutive decisions for the upstart Diamondbacks, who entered Tuesday 1½ games behind San Diego in the NL West.
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.
"I feel like I've hit my stride," Johnson said. "I think my numbers indicate that. Last year, I had a 5.00 ERA because my back was bothering me. I think I put a lot of things to rest when I came back here and I started pitching well."
Johnson was lighthearted during a 15-minute Chase Field news conference.
"I'm on some medicine," Johnson said. "I don't know if the hitters want to hear that. But I'm trying to make light of the situation."
When a reporter asked Johnson what drugs he was taking, Johnson replied, "Do you want some?"
The Diamondbacks have an off day Thursday, so they won't need to replace Johnson in the rotation for at least a week. Melvin said right-hander Edgar Gonzalez (3-2, 4.44 ERA) would be the leading candidate to take Johnson's turn down the line.