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Well, it’s not like anybody expected them to go up.

The season premiere of the pivotal 10th season of Fox’s American Idol fell in the ratings compared to last year.

Idol drew 26.2 million viewers, down 13 percent. Among the key adult demo, the show earned a 9.7 adults 18-49 rating, down 18 percent from 2010’s opener. The largest declines were among younger viewers, with teen viewership down 28 percent.

The 18-49 drop represents the lowest-rated premiere for Idol since its 2002 summer debut, and the biggest drop ever for the show between seasons.

Either way, the figures are below Fox’s panic line. Falling is fine, goes the mantra, as long as it’s not too big of a plunge — say, more than 20 percent.

This morning, Fox insiders say they’re relieved, noting reviews have been positive, fans seem to have embraced the new judges and the show went head-to-head with ABC’s Modern Family and doubled its rating — all despite losing Cowell.

The biggest test will come on Idol‘s new night, Thursday, when the reality series will face steeper competition and audiences should give the network a greater sense of whether they’re digging the new judges panel. More analysis and ratings after the jump…

But in one good sign from last night, the rating grew throughout last night’s two-hour telecast.

Idol utterly crushed Paula Abdul’s struggling CBS series Live to Dance (5.3 million, 1.0). But ABC’s Modern Family (11.1 million, 4.6) held up fine under the Idol pressure (here’s a clip of that walking-in-on-the-parents-having-sex scene). The second episode of ABC’s Off the Map (7.8 million, 2.1) slipped a bit from its lackluster premiere despite airing twice last week. The return of NBC’s Chase (4.3 million, 1.0) tanked. CBS tried out Blue Bloods (12.2 million, 2.0) at 10 p.m. and the show improved 17% on its Friday average.