HBO’s “Boardwalk Empire” returned Sunday night to a smaller audience for its fourth-season premiere, no doubt impacted by the big NFL game featuring New York’s Giants.

Nielsen estimates that the Prohibition era drama starring Steve Buscemi averaged 2.4 million viewers for its regular-timeslot telecast — down from the 2.9 million who watched the kickoff of the show in both 2011 and 2012. It’s a bit above the 2.3 million that “Boardwalk” averaged for all episodes a year ago.

By comparison to other HBO dramas, the first telecast of the most recent season of “True Blood” drew 4.52 million while “Game of Thrones” did 4.37 million and “The Newsroom” got 2.22 million.

“Boardwalk Empire” faced tough competition from the season premiere of TV’s top-rated “Sunday Night Football” on NBC, whose New York Giants-Dallas Cowboys matchup drew about 25 million viewers — and likely stole some male viewers in New York who may have otherwise tuned in to the HBO drama on its premiere night.

A replay of “Boardwalk” drew about 800,000 late Sunday, bringing the cume audience for the show to 3.2 million.