Westwood, a runner-up at the Masters and the British Open this year, suffered a calf injury at the French Open the first week in July. It has caused problems with swelling in his right ankle, and it reached a breaking point Friday.
"I will be out for as long as it takes to get better," Westwood said in a statement. "I am just hoping that it will be in time for me to play in the Ryder Cup."
Westwood, No. 3 in the world, had a chance to go to No. 1 in the ranking with a victory this week at the Bridgestone Invitational. He had his ankle taped for the second round and sputtered around to a 76.
Asked after his round what he could do besides tape his ankle, Westwood replied, "Sit on my back side for six weeks, like they keep telling me. It's the only way to improve it."
He withdrew from the Bridgestone Invitational later in the afternoon, appearing to take that advice.
"There's no strength in it," he said. "I don't have confidence in it, and then on the way down I'm finding it hard to hold my weight on it and then push off."
He will be replaced in the PGA Championship by Kevin Sutherland.
Westwood won the European Tour money title last year for the second time in his career, and the best player without a major has been closing in on one. He came within one putt of making the 2008 U.S. Open playoff at Torrey Pines, and the 2009 British Open playoff at Turnberry last year. He tied for third in the PGA Championship a year ago at Hazeltine.
His only victory this year came on the PGA Tour at the St. Jude Classic, a week before the U.S. Open. Westwood suffered the injury two weeks before the British Open, but still managed to finish second.
He has not played since then, and is not sure when he will play again.
"I'd like to be able to play my best and work my hardest, which I can't do," Westwood said after his round. "I can't even hit balls, you know? It's evident that I'm rusty since the Open because I haven't been able to practice. It's just a vicious circle, really."
His day wasn't quite over after the 76. After signing his card, his name was pulled for a random drug test.
"Be back in about three hours," he said to reporters, rolling his eyes. Now, it will be much longer than that.
Westwood already has locked up a spot on the European team for the Ryder Cup, leading the standings in money and world ranking points. The Ryder Cup will be Oct. 1-3 in Wales.