NBC Nightly News anchor Brian Williams will be taking a leave of absence to undergo knee replacement surgery, he announced Thursday on the air.

This will be his fourth surgery on the knee, the result of a high school football injury years earlier. “I’ve paid a very high price for what’s been confirmed to be the single worst New Jersey High School Catholic League football career in history,” Williams joked. The pain he has been experiencing, however, is very serious—even keeping him up at night. “Worst-case scenario,” Williams said, “if this doesn’t go perfectly well, which it will, it can’t hurt more than it hurts now.”

Williams will be off the air beginning next week. He may be gone anywhere between three weeks and three months; full recovery, on the other hand, takes from six months to a year.

During a recent appearance on The Late Show with David Letterman, Williams jokingly commented, “I want to come back almost a little too early gooned on medication. I think it will be great viewing.”

Lester Holt will serve as Nightly News anchor in his absence.