Why the 49ers will lose to the Seahawks in Week 18

The 49ers have won six games in a row. And if they win just one more, they'll get the No. 1 seed and a first-round bye in the playoffs.

Surely, they won't let Sam Darnold beat them at home with so much at stake, right? That would be humiliating. This is Purdy's former backup we're talking about. The guy who currently leads in the NFL in turnovers. The guy who turns into a pumpkin at the end of every season.

Unfortunately for the 49ers, their starting quarterback, Brock Purdy, has his moments when he falls apart, too. In fact, the only quarterbacks in the NFL with a higher interception percentage this season than Purdy are Tua Tagovailoa, who got benched, and Geno Smith, whose record is 2-13.

Dec 28, 2025; Santa Clara, California, USA; San Francisco 49ers quarterback Brock Purdy (13) throws a pass against the Chicag — Sergio Estrada-Imagn Images

Granted, Purdy has raised his game recently. Still, when the 49ers faced the Seahawks in Week 1, Purdy threw two interceptions. To be fair, he had turf toe. But, those picks were bad decisions, just as his pick-six was a few nights ago against the Bears.

So if the 49ers lose to the Seahawks at home on Saturday, this will be why: the Seahawks' defense is outstanding, and the 49ers' defense isn't good.

The Seahawks defense have given up just 18.1 points per game this season -- second-fewest in the NFL. When they faced the 49ers in Week 1, the 49ers scored a mere 17 points, and they had all of their starters.

What's more, the Seahawks defense has 44 sacks this season -- eighth-most in the NFL. And Purdy will have to face that pass rush most likely without future Hall of Fame left tackle Trent Williams, who pulled his hamstring on Sunday against the Bears. Hamstring pulls typically are multi-week injuries, particularly for 37-year-olds who weigh more than 300 pounds.

Meanwhile, Darnold gets to face the worst pass rush in the league. That's not an overstatement. The 49ers defense has recorded a league-low 18 sacks this season. On Sunday, they sacked Caleb Williams zero times, which is why he played so well.

Every quarterback plays his best against the 49ers. Even Jacoby freaking Brissett set an NFL record for completions in a game against the 49ers a few weeks ago.

In addition, Darnold gets to face a 49ers defense that has intercepted just six passes all season -- fourth-fewest in the league. Which means he probably won't turn the ball over against the 49ers.

Dec 28, 2025; Charlotte, North Carolina, USA; Seattle Seahawks quarterback Sam Darnold (14) looks to throw a pass while scram — Jim Dedmon-Imagn Images

Purdy has to face a Seahawks defense that has intercepted 17 passes this season. And he'll be under constant pressure. So his challenge will be much more difficult than Darnold's.

It's fair to expect the Seahawks offense to perform much better against the 49ers on Saturday than they did in Week 1 when they scored just 13 points against a defense that had Fred Warner, Nick Bosa and Mykel Williams. At the same time, it's tough to expect the 49ers' offense to perform significantly better against Seattle this time around without Trent Williams.

Don't be shocked if the 49ers lose this game. It's not a layup by any means.

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