Starting this week, a new era for prime-time cable news begins.

Tucker Carlson will assume Fox News’s 8 p.m. time slot on Monday, the first time in nearly two decades that Bill O’Reilly will not be kicking off Fox News’s prime-time lineup.

And Fox’s competitors are taking notice.

Mr. O’Reilly’s dismissal amounts to an enormous shift in cable news, and the TV industry was ablaze with talk late last week whether the move has the potential to open up a cable news war that for more than a decade has been dominated by Fox News.

It was already expected to be a big week for cable news. Though the 100-day mark for Donald J. Trump’s presidency is mostly symbolic, the cable networks were preparing to cover it the way only they can: as a TV spectacle. MSNBC was preparing a big week of programming, CNN invited its daytime anchor Jake Tapper to take over its 9 p.m. hour all week, and Mr. O’Reilly was supposed to return from a vacation to resume his role as the top-rated host in cable news.

It will be even bigger now. In addition to Mr. Carlson moving to 8 p.m., Fox’s afternoon opinion round-table show, “The Five” will move into the 9 p.m. slot, followed by Sean Hannity. It is an untested lineup, and some TV executives argued that Fox could be the most vulnerable it has been in years, particularly with MSNBC and CNN surging.