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Despite all the attention focused on the rather relentless parade of new series premieres on TV this week, there’s much at stake for some veteran skeins trying to fend off their own mortality.

Three of these mainstays — ABC’s “Grey’s Anatomy,” CBS’ “CSI” and NBC’s “The Office,” all with new lead-ins this year — air during the 9 p.m. Thursday hour that has in recent years been one of TV’s most competitive. While these programs aren’t in trouble, the cracks are surfacing.

Most of the offseason attention has gone to “The Office,” ever since talk was confirmed that its coming seventh season would be star Steve Carell’s last. Thoughts of visiting Scranton without running into Carell’s Michael Scott compounded concerns over a show most diehards felt had its poorest creative year — all things being relative — since its 2005 launch.

Though auds haven’t considered Thursdays must-see viewing on NBC for some time now, its comedy lineup has delivered worthy ratings in selected demos — with “The Office” leading the way. For a network in a pretty serious rebuilding mode following the failed Jay Leno primetime experiment — and how the commitment to it depleted the Peacock’s primetime pickings — a farewell to “The Office” even after some 150 episodes would seem too soon.

That’s why NBC made clear from the outset of the Carell chatter that the network wants “The Office” to continue without him. And NBC actually does have reason to feel upbeat.

For one thing, the impending Carell departure and rejigging of the show’s dynamic could be just what the series needs after its somewhat aimless sixth season. For another, the one place NBC is fairly strong is in up-and-coming comedies, with “Community” and “Parks and Recreation” looking like long-term laffers for the network, joining “30 Rock” (whose move to 8:30 p.m. potentially offers “The Office” a small boost).

Obviously, NBC would be even more at ease if “The Office” helped launch the freshman comedy airing behind it, “Outsourced,” into a hit.

By comparison, there’s been less public hand-wringing this summer over “Grey’s Anatomy,” also entering its seventh season, but it has begun a slow bleed of viewers that might raise more serious issues for ABC than “The Office” does for NBC.

Last nominated for a drama series Emmy in 2007, “Grey’s” has undergone criticism for its storytelling longer than “The Office” has, and the ratings slippage — a 6% decline in 2009-10 to a 5.3 rating among adults 18-49 and 13.8 million viewers overall — no doubt reflects that in part.

ABC can still point to strong viewership for the show — to be clear, any network would be happy to have “Grey’s” (last season’s No. 1 drama in 18-49) among its worries. But if you consider the importance of the show to ABC’s schedule, you can understand why the decline matters.

“Grey’s” not only accounts for a full hour of primetime as opposed to 30 minutes for a sitcom like “The Office,” it directly props up 10 p.m. sister series “Private Practice,” which remains one of the few successful dramas that ABC has been able to maintain over the past several years. (“Castle,” “Desperate Housewives” and “Brothers and Sisters” are the only other returning dramas on ABC’s fall slate.)

In fact, the demise of one-time 8 p.m. Thursday lead-in “Ugly Betty” and the failure of last season’s “FlashForward” to take hold, yielding this fall’s roll of the dice on faux documentary “My Generation,” have deprived “Grey’s” of a lead-in boost — once again extending the pressure of the show to keep generating its own audience as it ages.

CBS is facing even greater viewer loss with timeslot rival “CSI” — in its 10th season, the procedural dropped more than 20% in the Nielsens — but the Eye has less riding on it.

“CSI” doesn’t make or break Thursdays for CBS. At 10 p.m., third-year drama “The Mentalist” is as strong as ever. At 8 p.m., CBS offers two comedies — popular, battle-tested “The Big Bang Theory” followed by novice “S#*! My Dad Says.” The latter, even with William Shatner, doesn’t figure to help “CSI” much as a lead-in, but if things don’t work out, CBS can always return “Bang” to Mondays and move “Survivor” back from Wednesdays, where it formed one of the most successful ongoing two-hour combos with “CSI” in TV history.

Watching all this closely, of course, will be the people at CW, which has put its most promising new drama “Nikita” at 9 p.m. Thursdays, and Fox, whose gamble to move “Fringe” to the timeslot for its second season in 2009-10 is looking more and more shrewd. Neither figures to win during the hour, but don’t be surprised if more viewers, tired of the old, migrate to the new.