The 32-year-old American won this month's US Open after a playoff against Rocco Mediate before announcing that he needed reconstructive surgery on his left knee and would take the rest of the year off.

He told a teleconference on Monday he was not sure when he would make his comeback.

"We have to see how this thing heals. Everyone heals at a different rate," he said. "Some people are back to playing sports in six months, some are nine, some are 12."

Woods is wearing a straight-leg brace and will use crutches for three weeks because he cannot put weight on the leg.

He is host of this week's AT&T National at Congressional Country Club outside Washington DC but will not be attending the tournament.

"I would love to be there," Woods said. "Unfortunately, I don't think I can make it. Flying, unfortunately, swells up my leg pretty good. When I flew home from the procedure, it ballooned up a little bit.

Woods underwent an anterior cruciate ligament (ACL) reconstruction of his left knee.

"They did take a graft, basically a tendon out of my right hamstring, and implemented it into my left knee and made it my new ACL and they fixed a little bit of cartilage damage I had in there, and that was about it," he said.

Woods revealed he had taken a break at the end of last season after tearing the ACL to build up other muscles to support his golf swing.

"The reason why I took the long layoff is to develop my hamstring and my glute and my calf ... to basically give my leg more stability," said Woods, who tore his knee ligament after the 2007 British Open while running and took 10 weeks off at the end of the season.

"People have played without ACLs and have been very successful. Downhill skiers ski without ACL's, but they've got extreme sized glutes and hamstrings, and that's their checking mechanism. And I tried to do the same thing.

"I went through the rehab process and tried to get it ready for this year. It held up great," said Woods, who won his first three events in 2008 and was Masters runner-up before claiming his 14th major at the US Open.

"Unfortunately, as I kept playing on it, it became more unstable," he said. "The natural rotation of the golf swing without the ACL made it a little bit unstable, and it caused some cartilage damage because of that. I had that rectified after the Masters."

Woods took two months off after having arthroscopic surgery two days after the Masters and did not play competitively until the US Open two months later.

"That surgery I had after the Masters was to get me through the rest of the '08 season ... but, as you know, I developed stress fractures and decided to bag it for the year."

He said he was looking forward to playing without pain.

"I've been trying to adjust over the years to alleviate some of the stress I do put on my left leg," he said. "But, basically, my left knee's been sore for 10, 12 years, so it will be nice to finally have a healthy leg.

"The doctors have assured me that my long-term health will be a hell of a lot better than it's been over the past decade.

"So I'm really looking forward to that and not having pain after I'm playing and while I'm playing."