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 top-rated host in cable news — and with 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 had 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.