Total viewership of the NFL is down 7.5 percent in 2017 compared with the first six weeks of 2016.

An average of 15 million people watched games for the first six weeks of 2017 compared to 16.2 million through Week 6 last season.

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.

Compared with the 2015 season, NFL ratings are down 18.7 percent. The decline is sharp but smaller when considering the overall fall in television viewership due to cord cutting of cable.