Through the first six weeks of the NFL season, total viewership of games is down 7.5 percent, compared with the first six weeks of 2016.
An average of 15 million people watched games for the first six weeks this year, compared with 16.2 million people through Week 6 last season, according to metrics from Nielsen.
An NFL spokesman didn't immediately return a message from ESPN seeking comment.
In the past week, Credit Suisse lowered its price targets on both Fox and CBS stock, citing NFL ratings declines.
"If ratings don't improve materially, we see a potential headwind to domestic advertising revenues," the investment bank's analyst Omar Sheikh wrote of Fox on Thursday.
On Monday, Sheikh estimated third-quarter revenue for CBS would be off 3 percent, versus the previously projected 1 percent, due to "soft ratings for both the summer [programming] and for the start of the NFL season."
Through six weeks (seven games), ESPN's Monday Night Football viewership is averaging 11.2 million viewers, a 6 percent increase through Week 6, compared with last year.
Compared with the first six weeks of the 2015 season, NFL ratings are off 18.7 percent -- a sharp drop, but a smaller decline when considering the general overall fall in television viewership due to people dropping their cable packages.
Last year, the NFL attributed part of its viewership decline during the first nine weeks of the season to competition from presidential election coverage. Viewership was indeed better for the second half of the season, after the election, but it still finished down 8 percent, compared with 2015.
This season's challenges also include player protests during the national anthem, which have pitted the NFL against President Donald Trump, who again tweeted Wednesday that players should stand to honor the flag.
The NFL has decided that it will not force players to stand for the playing of our National Anthem. Total disrespect for our great country!
— Donald J. Trump (@realDonaldTrump) October 18, 2017
Players and owners met Tuesday in New York to discuss how best to deal with their public platform and work together to support the fight against social injustice. The meeting led to further conversation on the matter but no immediate resolution.