NEW YORK - NBC News anchor Brian Williams will be off the air for a few weeks soon for surgery to replace a knee that was damaged in a high school football game decades ago.

Yet in what may be a sign of increased competitiveness in an evening news ratings race that Williams has dominated for several years, NBC News would not say Friday when Williams would go under the knife. Spokeswoman Erika Masonhall would say only that Williams will anchor "Nightly News" on Monday.

Williams discussed the knee replacement surgery Thursday in a "Nightly News" segment about the procedure by Dr. Nancy Snyderman.

"If you don't see me around here for a while, this is the reason why," he said.

Lester Holt will anchor the nightly news while Williams is away.

Williams, 54, said he's had three previous surgeries on his right knee but the pain was becoming more of an issue. When it began costing him sleep, Williams said he realized it was time to take care of it.

"If this doesn't go perfectly, it can't hurt any more than it does now," he said.

Snyderman said most people who have knee replacement surgery miss between three weeks and three months of work, depending on the physical nature of their jobs. Full recovery takes between six months and a year.

Williams joked during an appearance on David Letterman's "Late Show" two weeks ago that "I want to come back almost a little too early gooned on medication. I think it will be great viewing."

His newscast showed a rare sign of ratings vulnerability last week. The Nielsen company said ABC's "World News" beat NBC among the 25- to 54-year-old demographic that most news advertising sales are based on, breaking an NBC winning streak of 243 consecutive weeks. NBC won when viewers of all ages were counted.