For Fox News, there is one big question: What does life after Bill O’Reilly look like?

The abrupt and once-unthinkable defenestration of Mr. O’Reilly on Wednesday leaves Fox News without the No. 1 rated host in cable news — and a potentially far-reaching shake-up in the channel’s vaunted evening lineup.

Tucker Carlson, a conservative provocateur who joined Fox News’s prime-time ranks only three months ago, has been tapped to replace Mr. O’Reilly at 8 p.m. Eastern, beginning on Monday, the network said. Mr. Carlson has become, seemingly overnight, one of the network’s most vital players, a remarkable turnaround for a pundit whose bow-tied heyday seemed behind him.

Fox News’s afternoon ensemble show, “The Five,” which features a chatty round table of hosts, will take over Mr. Carlson’s 9 p.m. slot.

If the exit of Mr. O’Reilly was not enough to rattle the executive suites at Fox News, the network must now navigate a drastic on-air overhaul at a rare moment when the channel is facing a challenge from its rivals.

Fox News’s 15 years of prime-time dominance is a mantra inside the network. But last month, Rachel Maddow on MSNBC surged past Mr. Carlson at 9 p.m. in the coveted 25-to-54-year-old age bracket. CNN just announced a prime-time test next week for Jake Tapper, whose sharply critical analysis of President Trump has made him a news media star of the moment.

The prospect of Ms. Maddow and Mr. Tapper at 9 p.m. would amount to a pincer move by Fox News’s rivals to dethrone their longtime Nielsen dominator.

This will be Mr. Carlson’s third time-slot change in four months — an unheard-of fluidity at Fox News after years of a more-or-less static lineup. Since January, Mr. Carlson has been tasked with replacing three departing Fox stars: Greta van Susteren, Megyn Kelly and, now, Mr. O’Reilly.

The only remaining member of Fox’s longtime four-hour stretch of nighttime programming is Sean Hannity, who is drawing huge ratings and will continue to command his 10 p.m. slot.

Mr. O’Reilly’s ratings dominance was no small matter. Before the New York Times report on April 1, he averaged nearly four million viewers, more than 700,000 viewers better than Mr. Carlson’s second-place 9 p.m. show.

Even before Mr. O’Reilly’s exit was announced, senior Fox News executives were scrambling to lock down a revamped slate of evening anchors, with Rupert Murdoch, the head of Fox News, closely involved in the discussions, according to a person told of the network’s internal deliberations. The network was eager to piece together a lineup that could be up and running by Monday, when Mr. O’Reilly had been scheduled to return from a vacation to Italy.

Fox News also said it would grant Eric Bolling, who has been a substitute anchor for Mr. O’Reilly and a host of “The Five,” his own afternoon program at 5 p.m. beginning on May 1.

In its new prime-time incarnation, “The Five” will add a regular panelist: Jesse Watters, a protégé of Mr. O’Reilly who recently interviewed Mr. Trump. Mr. Watters is best known for prankster-style antics and was heavily criticized last year for a segment that trafficked in offensive stereotypes about Asian-Americans.