The NFL's declining TV ratings, blamed by some on the national anthem controversy, could soon take a financial toll on the broadcasters that have spent tens of billions dollars for the right to air pro football, investment analysts say.

Through the first five weeks of the season, ratings for the Sunday night game on 21st Century Fox's (FOXA) Fox Sports have slumped 7 percent from a year ago. NFL viewership at CBS (CBS), the parent company of CBSNews.com, has plunged 17 percent for Sunday match-ups and 8 percent on Thursday. The audience for NBC's "Sunday Night Football" game broadcast has dropped 4 percent.

ESPN's "Monday Night Football" is bucking the trend, posting a 6 percent gain for the season to date. The sports cable network is owned by Disney (DIS).

"Probably it's a little early to use the word 'worry,' but at this point I think it's something that deserves close attention," said Tuna Amobi, an analyst with CFRA who follows the media sector. "To have now two consecutive years of declines in NFL ratings -- that is something that begs a lot of questions … and could have significant financial implications for the ratings."

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Despite the ratings slump, the NFL remains by far the most popular U.S. sport. Since the start of the season, 17 of the top 20 shows on TV have been NFL games. NBC's "Sunday Night Football" remains the most watched show in Prime Time.

."NFL games remain the most valuable programming on television and continue to deliver massive audiences across all windows," NFL spokesman Brian McCarthy said in an email.

Yet those large audiences that advertisers covet must be balanced against the steep price tag of beaming NFL games. CBS, Comcast, and Fox have a $27 billion deal with the NFL for the right to broadcast games through 2022. ESPN reportedly pays $1.9 billion per year for the rights to "Monday Night Football," a 73 percent increase over the previous contract. NBC and CBS also signed a $900 million deal in 2016 for the rights to broadcast Thursday night games.

Whether the NFL ratings slide has reached the threshold where the networks would have to provide advertisers so-called make-goods -- essentially free commercial time -- isn't clear. Advertisers pay rates for TV commercials based on guarantees from networks regarding the number of viewers a show will attract.

But Wall Street is keeping a close eye on how the ratings decline could affect broadcasters. Credit Suisse analyst Omar Sheikh said in a note this week to clients that if ratings "don't improve materially" for Fox, "we see a potential headwind to domestic advertising revenue" in the current fiscal quarter. He cut his 12-month price target on the company's stock from $37 to $35.

Sheikh also trimmed his per-share earnings forecast for CBS for the quarter to $1.05 following what he called "soft ratings for both the summer schedule and for the start of the NFL season."

Officials from Fox Sports couldn't immediately be reached for comment. A spokesman for ESPN referred questions about TV ratings to the NFL. CBS and NBC Sports declined to comment.

"It's a Catch-22 type situation," Amobi said. Broadcasters "are getting a lot of promotional value from these games, and maybe investors are going to ask more questions the next time these [broadcast rights] renewals come around."