Four games, zero sacks. Purdy's streak dies in Seattle.

Reason 1: Seattle sacks Purdy twice

Brock Purdy has thrown 113 passes this season and has not been sacked once: not against the Rams, Miami, Arizona or Denver. Seattle has 11 sacks in the same four weeks, four of them last Sunday against the Chargers.

Hawk Blogger's numbers frame the collision: "The Seahawks defense ranks 4th at creating pressure (36.9%). The 49ers offense ranks 2nd in pressure rate allowed (20.3%) and has not given up a sack (0.0%, 1st)." Something gives.

History says it is Purdy. Against Mike Macdonald, "He has 6 touchdowns and 9 interceptions over his career." And Leonard Williams, held out of practice Thursday with an ankle injury, is cleared to play. Our call: Seattle sacks Purdy at least twice.

Reason 2: McCaffrey runs for less than 60

Christian McCaffrey's four games: 68, 23, 75 and 52 rushing yards. Half the time he is already under 60.

Sunday he loses his lead blocker. Fullback Kyle Juszczyk is out with a knee injury, and "The 49ers run 21 personnel (a fullback and a running back) more than any offense in the NFL."

Seattle allows 3.6 yards a carry and, per Hawk Blogger, ranks 1st in run defense DVOA. Our call: McCaffrey runs for fewer than 60.

Reason 3: Seattle converts 40% on third down

This one is aimed straight at our weak spot. Seattle's offense is 14 of 45 on third down, 31 percent: 2 of 11, 6 of 13, 3 of 9 and 3 of 12. San Francisco converts 57.5 percent.

The window is Nick Bosa's absence. He is out, and the 49ers defense already "ranks 24th at creating pressure (25.8%)". Sam Darnold gets time. Our call: Seattle converts at least 40 percent of its third downs.

Unbeaten, and still the underdog

San Francisco is 4-0, scoring 30.5 points a game. The market still makes Seattle the favourite: our captures had Seattle by 3.5 from September 30 through October 4, and by 3 every day since.

Our projection adds up every player's projected points and gets Seattle 27.7, San Francisco 22.0. That's Seattle by 5.7: more than the number, and a Seattle win.

The last time these teams met, the 49ers' season ended. Hawk Blogger: "The 49ers saw their season end in humiliating fashion in this building." Across the last two meetings San Francisco "averaged 4.5 points, without a single touchdown."

The case against us

Purdy is rolling: 11 touchdowns, one interception, and 20 of 22 against Miami. Macdonald himself: "I'd have a hard time underrating him. He's a hell of a player."

And Seattle's offense can go missing. It gained 286 yards against the Chargers, and Hawk Blogger has its run game ranked 30th in rush DVOA, with Zach Charbonnet still out. If Seattle stalls on third down again, Reason 3 goes first, and the pick may follow it.

These are Friday designations. Sunday's inactives come after this was written.