Kim O'Reilly, CBS Sports

Three weeks ago, it looked so simple. The San Francisco 49ers looked like the top team in the NFC with the Kansas City Chiefs the top team in the AFC.

Why play the rest of the season? It was so obvious.

Then things changed in a big way — as they always do in the NFL. The 49ers have lost three straight games and their supposed dominant defense can't stop anybody, while Brock Purdy has looked like anything but Joe Montana in the clutch. The 49ers are now 5-3 and spiraling downward in a big way. 

Then the Chiefs and Patrick Mahomes didn't score a touchdown against the Denver Broncos Sunday, the same Denver team that gave up 70 to the Miami Dolphins. The Chiefs are now 6-2 and, aside from a game or two, have looked ordinary on offense, which is not what we expect from Mahomes and gang.

In less than a month, the top of the league has been turned upside down — and the NFL wouldn't have it any other way. Just when we think the dominant teams are emerging, we get what happened to the 49ers the past three weeks and the Chiefs this past Sunday.

So which of the two should be more concerned? The 49ers. For all the talk about Purdy not playing well in crunch time the past two weeks, turning it over, the bigger concern for San Francisco has to be the defense. In successive weeks, Kirk Cousins and Joe Burrow went a combined 63 of 77 for 653 yards, five touchdowns and one pick. That's called putting out the cutlery set as they took the knives and carved you to shreds.

The Chiefs just had a bad offensive game against the Broncos this week, but they've had some lulls at times on that side of the ball earlier this season. It just hasn't been as crisp as in year's past.

The Chiefs are down to sixth in my Power Rankings this week with the 49ers down to 11th. For San Francisco, it's a really fast drop from all that talk a month ago about going undefeated.

Then again, that's the NFL — unpredictable, wild, big swings and teams going up and down every week of the season.

If you have a handle on this season, let me know. When the 49ers can't play defense and Patrick Mahomes can't score a touchdown against a team that gave up 70 earlier this season, I give up.

Biggest Movers
9 Titans
7 Falcons
1 Eagles It wasn't always pretty against the Commanders, but they got it done in the second half. Now they face a big one with Dallas this week. 1 11-6-0
2 Ravens They found a way against Arizona, but the passing game took a step back. That's a concern for this group. 1 13-4-0
3 Jaguars They've won five straight to right things as they head to the bye. They are definitely in the conversation for best team in the AFC. 1 9-8-0
4 Cowboys They've won two straight as they head to Philadelphia for a big game with the Eagles. Dak Prescott was really good against the Rams, which has to carry over this week if the Cowboys are to beat the Eagles. 2 12-5-0
5 Dolphins Tua Tagovailoa owns Bill Belichick after beating him for the sixth straight time. Now they face a big game against the Chiefs in Germany. 2 11-6-0
6 Chiefs What happened to the offense against the Broncos? It had to be more than just Patrick Mahomes being sick. They can't play that way against Miami - or else. 5 11-6-0
7 Bills They got the offense back on track against the Bucs, but now they face a tough road prime-time game against the Bengals. That will be another proving game for this inconsistent group. 6 11-6-0
8 Bengals Coming off their bye, they rocked the 49ers. Joe Burrow was back to being Joe Burrow after struggling with a calf injury since the summer. 6 9-8-0
9 Lions They bounced back in a big way against the Raiders, although they did leave points on the field. Against a good team, that will kill them. Jahmyr Gibbs was outstanding. 1 12-5-0
10 Seahawks At 5-2, they keep finding ways to win games - even when it doesn't look good. The defense is showing real signs of improvement. Getting Leonard Williams from the Giants helps. -- 9-8-0
11 49ers They've suddenly become a team that can't stop anybody. What's wrong with the defense? They have the bye to figure it out this week. 6 12-5-0
12 Jets They have a winning record. Yes, a winning record. They beat the Giants in a game they shouldn't have won, but they found a way. 3 7-10-0
13 Browns With P.J. Walker playing quarterback, they are limited in a big way on offense. It's time Deshaun Watson gets back out on the field. 4 11-6-0
14 Chargers That Chargers team we saw against the Bears is the one we expect to see with all that talent. But consistency is always an issue with that team. 4 5-12-0
15 Saints The got Derek Carr back going good against the Colts. They needed that to turn their season back around. 6 9-8-0
16 Steelers The offense regressed against Jacksonville and now Kenny Pickett is hurt. It's amazing Mike Tomlin has this team with a winning record. 5 10-7-0
17 Texans C.J. Stroud and the offense struggled for the first time this season in losing to the Panthers. That's to be expected for a rookie quarterback, but now it's time to see how they respond. 1 10-7-0
18 Titans Will Levis lit it up in his first NFL start, throwing four touchdown passes. Now he will face a more challenging start this week against the Steelers on the road. 9 6-11-0
19 Falcons They have a quarterback problem. Desmond Ridder was benched, which means it could be time for Taylor Heinicke to take over. 7 7-10-0
20 Rams For the first time this season, they got blown out as the Cowboys beat them up Sunday. The defense had a rough day. 3 10-7-0
21 Buccaneers They've lost three straight games to dampen what was a solid start to their season. But in their division, they are far from done. 1 9-8-0
22 Colts They compete in games with Gardner Minshew, but they can't overcome all the mistakes. They are falling into a big hole in the division. -- 9-8-0
23 Vikings With Kirk Cousins out for the year, they are done, which is why they are down this far. He was playing outstanding football before tearing his Achilles against the Packers. There is no quarterback who can save them now who would be available or who is on the roster. 4 7-10-0
24 Commanders They had their chances against the Eagles, but they have to start winning games. Could Ron Rivera be in trouble? 1 4-13-0
25 Raiders For a team coached by Josh McDaniels, they are impotent on offense. They just can't score enough. It's ugly. 1 8-9-0
26 Broncos That was an impressive showing against the Chiefs by their defense. That unit has made big strides. Holding the Chiefs without a touchdown is impressive and the offense was solid. 5 8-9-0
27 Patriots After a big win against the Bills, it was back to reality against Miami. They just aren't a good team - even with Bill Belichick as coach. 2 4-13-0
28 Bears Tyson Bagent can do some good things, but they need Justin Fields back. Bagent looked ordinary against the Chargers. The defense was awful against Justin Herbert. -- 7-10-0
29 Packers They are in a free fall right now, and they can't seem to stop it. This season has spun out of control in the worst way. Are they done? It looks like it. 3 9-8-0
30 Giants They had no business losing to the Jets, even with a third-team quarterback who didn't even try to pass. That late-game situational football was a disaster. -- 6-11-0
31 Cardinals They need to get Kyler Murray back playing again, and it might happen this week. The Josh Dobbs era appears to be over. 2 4-13-0
32 Panthers They won for the first time in beating the Texans, but it wasn't easy. The offense did show some signs of life, which is a good thing. -- 2-15-0