PHOENIX -- Four-time NL Cy Young Award winner Randy Johnson will have arthroscopic knee surgery Thursday and miss three-to-six weeks.

The Arizona Diamondbacks left-hander, who was 24-5 last season, won his first game in three decisions Sunday after spending two weeks on the disabled list.

The operation in Phoenix will be performed to clear out debris that caused an inflammation in his right knee.

		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).

"We're hoping the procedure will eliminate the cause of the swelling," Diamondbacks manager Bob Brenly said Wednesday night. "He's taking this about as well as you'd expect. I mean, this is one of the great competitors in our game."

Johnson was not at the ballpark, where the Diamondbacks are playing the Florida Marlins, and the team said he would not be available for comment.

The Diamondbacks said head team physician Michael Lee would perform the cleanup, but refused to divulge the time or location.

"Really?'' Arizona first baseman Mark Grace said about the news. "That's really bad."

Head team physician Michael Lee, who will perform the cleanup, said it should take no more than 45 minutes. Johnson will be on crutches for a few days and then begin rehabilitation.

Johnson had an MRI exam on the knee about two weeks ago, and the decision to operate was made after a second MRI exam Wednesday.

'It shows, No. 1, a lot of fluid in the knee, which we already know," Lee said. "The second thing it shows are some small, cartilaginous pieces -- basically some small, loose bodies that have fragmented off a little bit and are probably causing the irritation."

Lee found no evidence of ligament damage or problems with the meniscus cartilage, but discovered one area that he said would require evaluation during the procedure.

He said the debris could stem from a knee operation performed after the 1991 season, when Johnson was with Seattle. It involved a patellar shaving to remove a rough area on the back of the kneecap.

Johnson was not immediately placed on the DL, but Brenly said rookie Brandon Webb will replace Johnson in the rotation and start Friday's game, the opener of a home series against Atlanta.

Webb pitched seven scoreless innings and beat the New York Mets 6-1 on Sunday in his first major league start.

Webb struck out 10 -- a Johnsonesque performance -- before Johnson pitched the second game of a doubleheader at Shea Stadium.

Johnson struck out 12 New York Mets on Sunday -- his 188th double-digit game -- and beat the Mets 7-3.

But he acknowledged afterward that he still wasn't in his old form in giving up nine hits and two runs in six innings.

Curt Schilling, the right-hander in Arizona's two-ace tandem, went on the 15-day DL after having an appendectomy in St. Louis on April 20. But he will be back this weekend and take the ball Saturday against the Braves.

"We've been through this in varying degrees the last two years, and it's part of the game," Brenly said. "The throwing motion is very unnatural to begin with."

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.

"That could have been frustration more than anything else," Brenly said.