Week 5 preview: The 49ers walk into the loudest building in football

Week 5 is when the league stops lying to you. Four games in, the tape has a body count. Everybody's got tendencies on film now, everybody's got a weakness the whole league has seen, and the teams still winning are the ones whose locker rooms are holding the line. That's not analytics. That's football.

Look, the game everybody's circling is San Francisco at Seattle. The Niners are 3-0 with 98 points on the board, and the Seahawks are favored by three at home because that building eats undefeated teams for breakfast. You know what a 3-0 team with a chip on its shoulder looks like? Me neither, because we haven't learned anything about these Niners yet. Blowouts don't teach you anything. Seattle will.

Bills at Rams is the one the analytics guys are going to overthink. Buffalo's favored by 2.5 in Los Angeles, and I keep hearing about efficiency numbers. Forget the numbers. Ask the simpler question: which locker room has been in this exact spot before and didn't flinch? That's the bet. Buffalo's been the almost-team for years, and almost is a heavy coat to wear into somebody else's dome.

Now the Eagles at Jacksonville. The Jags are favored at home over the defending whatever-you-want-to-call-them, and that line tells you the market thinks Philly's locker room is doing math nobody's saying out loud. I don't do anonymous sources and I don't do locker-room gossip. But I'll say this: teams that are right don't get points at home against teams that are wrong.

Ravens at Falcons is the bully game. Baltimore's laying six on the road in a dome, which is the market's way of saying Atlanta's defense is about to get a pop quiz it didn't study for. And Broncos at Chargers? Denver's 2-1 and favored, and I love a young team that travels well. Road favorites are a character test. You find out who leads when the crowd is booing.

Here's what I'm watching that nobody's writing about: the fullbacks and the gunners. Week 5 is when the working class of the roster starts deciding games — field position, hidden yardage, who wins the ugly snaps. The pretty teams are all pretty in September. October belongs to the grinders.

Verdict: By Monday morning we'll know if the 49ers are actually 3-0 good or just 3-0 lucky, and the Eagles will have told us everything about their season without saying a word. I'm taking Seattle to end the unbeaten talk and Jacksonville to keep Philly's questions coming. The locker rooms are talking. Listen.