That means NBC’s second act in late night, Jimmy Fallon, is also getting a much weaker lead-in than Mr. O’Brien did in the same hour a year ago, and as a result is falling behind his CBS competitor, Craig Ferguson, in audience totals.

NBC rightly points out that both its late-night stars remain more popular with younger viewers who are more valuable in selling to advertisers, but in the past NBC’s late-night hours were dominant across the board, not just with narrower audience segments.

And all of this is playing out against a backdrop of reports that General Electric is in talks with Comcast to sell NBC Universal.

Image On "The Tonight Show," the host, Conan OBrien, going for laughs with his guest, Hank Azaria. Credit... Margaret Norton/NBC

Looked at in isolation, Mr. Leno has been doing everything NBC expected of him.

His ratings, after a big first week, have leveled off to about five million viewers a night (though some nights have been much lower) with a 1.5 to 2 rating in the category NBC identifies as all-important, viewers ages 18 to 49, the group many advertisers want to reach.

Though most 10 p.m. shows with those kinds of numbers get canceled, NBC has said from the beginning that it could accept much lower ratings because of the enormous cost savings of Mr. Leno’s show versus expensive hourlong scripted dramas. The network guaranteed advertisers that it would average only a 1.5 rating.

The network’s performance over all has not shown signs of a comeback. While somewhat propped up so far by professional football on Sunday night, NBC has not added any standout new shows. The new drama “Trauma” has already faltered, and a promising new comedy, “Community,” struggled last week when it was moved to a new 8 p.m. time period.