Luke Falk, a Titans pick in the 2018 draft who the Jets picked up off waivers in May, played reasonably well subbing for Siemian, but no one is expecting him to pull a Kurt Warner and go from scrub to star. The line quickly jumped up about 5 more points on the news.

It makes no small difference that the Patriots have 30- and 43-point wins on their ledger and haven’t given up a touchdown.

Yet bettors don’t seem scared of either big line. FanDuel said 96 percent of its bets so far have come in on the Patriots and 92 percent on the Cowboys. Sportsbetting.ag said their figures were about 80 percent and 70 percent.

The general public seems to love these two big favorites. But Adam Burns, the sports book manager for Sportsbetting.ag, said: “Our smarter bettors, they’re not really touching it. When the line gets get past 14 or 17, a lot of good players lay off. You never know if there’s going to be a lucky touchdown late.”

Some cagier gamblers might wait until closer to game time to bet the underdogs. “We will see some sharp money against those teams,” Sheeran said. “I don’t expect it to move the line, because of the sheer weight of public money.”

The flow of money on both favorites has been helped by the Cowboys and Patriots being perhaps the two most popular teams in the N.F.L., bringing in even more bets from casual fans.

So which way should you bet this week? It turns out that the 11 favorites of 20 or more points since 1985 were only 2-9 against the spread, Sportsbetting.ag said. Sounds like an underdog bet may be the smart play.