PHOENIX -- Four-time NL Cy Young Award winner Randy Johnson underwent successful arthroscopic knee surgery Thursday afternoon, his first operation in seven years.

"According to all reports the surgery went as expected," Arizona manager Bob Brenly said. "We're very optimistic this thing will move along the way we hoped it would."

		Thurs., May 1 At 39 years old, Randy Johnson's knees have plenty of wear and tear, including previous surgeries. The D-Backs are smart to have this taken care of now, early in the season, so Johnson can return healthy and strong. I never had knee surgery during the season, but I underwent three offseason knee operations. And if a pitcher isn't careful, he can risk the health of his arm. Knee surgery is tough for a pitcher, because your arm feels great. But if you return too soon, you can overcompensate, which can lead to bad mechanics. There's the classic story of Dizzy Dean getting hit in the toes with the baseball, altering his delivery and then hurting his arm. And it cost him his career. That can also happen when a pitcher hurts his knee, due to favoring it so you don't land on it so hard -- and all of a sudden your shoulder is getting tender. It's crucial for a high-performance pitching machine like Johnson to be completely healthy before he tries to come back. Johnson is in good shape and he's a diligent worker, and he'll tackle his rehab like the competitor he is. So I wouldn't be surprised to see him beat his three-to-six-week recovery timetable. No question, the D-Backs need him to make a run at the postseason. There's no way anyone can replace a Randy Johnson. But Brandon Webb struck out 10 in his major-league debut Sunday, so Arizona hopes he can hold down the fort until the Unit is back. It's early, but the D-Backs don't want to lose much more ground in the NL West race (they trail the Giants by eight games).

Johnson, who was 24-5 last season en route to his fourth straight Cy Young Award, won his first game in three decisions Sunday after spending two weeks on the disabled list.

Johnson had an MRI on his right knee about two weeks ago, and the decision to operate was made after a second MRI on Wednesday showed loose particles causing inflammation in the left-hander's knee.

Head team physician Michael Lee removed the particles, the largest of which was about the size of a fingernail, in a brief procedure Thursday. Lee also repaired a small meniscus tear and drained fluid from the knee.

Johnson is expected to miss three-to-six weeks, though Lee said Thursday night the three-week time frame is "overly optimistic."

Johnson will be on crutches for a few days before beginning rehabilitation.

"The first MRI showed something was in there," Brenly said. "But anyone who's had knee surgery or knee problems know it's not totally debilitating. It's just a matter of where those loose particles choose to settle. The second MRI subsequent to his outing in New York indicated the floating particles weren't in a good place.

"For a guy like Randy, with as much effort as he expends on the mound and the stress he puts on that joint, I didn't imagine him being able to continue. By getting it taken care of now, optimistically we can have him back in a month."

Brenly said he hadn't spoken to Johnson and probably would not do so until Friday.

"I thought it would be best to give him today to rest and recover," he said.

Lee said Wednesday the debris in Johnson's knee could step from a knee operation performed after the 1991 season when the left-hander was with Seattle. That surgery involved a patellar shaving to remove a rough area on the back of the kneecap.

Lee performed a similar shaving on the underside of Johnson's patella, as well as the top of his knee. Looking back to Johnson's first MRI, Lee said there had been "no indication that (surgery) was the best way to proceed."

Brenly has been juggling his starting rotation for the past two weeks. Right-hander Curt Schilling has been on the disabled list since having his appendix removed April 19, and Johnson also missed a pair of starts.

Schilling will return to the rotation Saturday against the Atlanta Braves and rookie Brandon Webb, who beat the New York Mets 6-1 in his first career start, will replace Johnson in the rotation.

Webb, who pitched seven scoreless innings and allowed just three hits with 10 strikeouts in the first game of a doubleheader, was sent back to Triple-A Tucson after the game to make room for Johnson's return from the DL.

Johnson struck out 12 -- his 188th career double-digit game -- and beat the Mets 7-3 in the second game.

"You can't replace a Randy Johnson or replace a Curt Schilling," Brenly said. "You need to hope the guys you're putting into the rotation do what they're capable of doing. Brandon Webb isn't replacing Randy Johnson. He just needs to be Brandon Webb."

Another potential problem with the rotation arose Tuesday night, when right-hander Byung-Hyun Kim said after a 7-5 loss to Florida that he aggravated his sore right ankle -- hurt by the barrel of a broken bat on April 14 -- on a pickoff move.

Kim wasn't sure about being able to make his next start, scheduled Monday, but Brenly downplayed that concern after an MRI on Wednesday showed a severe bone bruise but no more serious damage.