The 54 year-old host is expected to miss several weeks with Lester Holt filling in beginning mid-next week.
Williams said he injured the leg years ago while playing high school football.
“I’ve been in pain for 35 years,” he said. “I reached a point recently in the last 12 months where I decided this has to happen and to be mathematical about it.”
Williams took over as NBC Nightly News anchor in 2004.