DENVER — Diamondbacks manager Bob Melvin is in no hurry to rush Randy Johnson back to the big leagues, especially after the Big Unit struggled through a rehab start.

Johnson went four innings and allowed five runs and eight hits in his first start with the Triple-A Tucson Sidewinders on Thursday.

The 44-year-old lefty is trying to come back from his second back operation in less than a year. Johnson is scheduled to make another start Tuesday in Tucson, Ariz. He went on the disabled list Sunday, a move retroactive to March 22.

"I've never been married to a certain date," Melvin said today when asked when Johnson would return to the Diamondbacks. "We've forecast certain dates, but if he feels like physically he needs another one then, depending on how many pitches he throws, we can do that as well."

Johnson left the game Thursday against Colorado Springs after throwing 78 pitches and giving up two walks. He also struck out one.

"I talked to him yesterday for a while," Melvin said. "He felt like physically he was out there more than four innings. We'll see what next time out holds as far as pitches and innings. If we need to get him another one to get ready, we'll do that."

Johnson gave up six earned runs over eight innings in three appearances this spring for Arizona.