But Ms. Kotb, a longtime fan favorite for her freewheeling segments with Ms. Gifford, has provided more than a steadying hand, and may even have given the show a boost. NBC has defeated its ABC rival “Good Morning America” every week since Mr. Lauer was removed, something it had not done for four consecutive weeks in more than five years.

The “Today” lead has narrowed — the program averaged around 4.4 million viewers the week before Christmas, about 54,000 viewers more than “G.M.A.” — and there is some uncertainty whether the lead is sustainable. Further, all three morning shows experienced rating drops last year.

A surprise boon from an emergency host isn’t new at NBC. When Brian Williams was suspended for six months in 2015 from his evening newscast after he was caught embellishing stories about a Middle East reporting assignment, Lester Holt replaced him and the show’s viewership figures were strong. Mr. Holt became the permanent replacement later that year.

Likewise, the departures of Charlie Rose from “CBS This Morning” and Mr. O’Reilly from Fox News after public allegations of sexual misconduct have done little to disrupt either network. Fox News is still No. 1 among cable news networks in prime time, and the ratings for “CBS This Morning” have held steady while Gayle King and Norah O’Donnell continue in Mr. Rose’s absence. (CBS has not yet hired a replacement.)

Ms. Kotb cut her teeth as a local reporter and anchor, including a six-year stint for the CBS affiliate in New Orleans. She joined NBC in 1998 as a correspondent for “Dateline.”