Coaches like to divide an NFL season into quarters. It’s an easy way to chart a team’s progress over a long, grueling season and to compare its performance over time. 

Offensively, the Bills have put together two radically different four-game periods, especially in the passing game. After four games, Josh Allen was on pace for the top statistical season in team history and on the short list for MVP. 

There’s no longer any serious discussion about Allen as an MVP favorite. Among Buffalo fans, there are concerns about a regression. Which is the real Josh — the guy who lit up the league in the first month, or the quarterback who has looked a lot like the Allen of a year ago during the last four games?

The statistics are fairly alarming. Over the first four games, Allen completed 71 percent of his passes for 1,326 yards, 12 touchdowns and one interception. He was near the top in the league passing charts and on pace to shatter franchise records in almost every meaningful passing category. 

Over the ensuing four games, the drop has been precipitous. Allen has completed 63 percent for 846 yards in his last four. That’s a drop of more than 100 yards a game. He has four TD passes and four picks in that stretch. His yards per pass attempt has plunged from 9.0 to 6.6 in the last four. 

After four weeks, Allen had a passer rating of 122.7, which was on pace to shatter Jim Kelly’s team record of 101.2 and to challenge Aaron Rodgers’s NFL record of 122.5

Since then, Allen’s passer rating has been around 77, roughly his average rating over his first two seasons. He has been trending back down, and the most troubling trend of all is the lack of chunk passing plays down the field. 

Over the first four weeks, Allen was killing teams with big throws. The Bills had 21 pass plays of 20 yards or more through four weeks, four completions that went for 40 yards or more. In the last four games, that figure has dropped to just four. Yes, one 20-yard completion per game on average.

Stefon Diggs had eight such receptions in the first quarter of the season, including two of more than 40 yards. He has no catches of 20-plus yards in the second four games. Zero.

Diggs has been very productive overall. He has 54 catches for 695 yards (second in the NFL). He’s on pace to break Eric Moulds’s team records for receptions (100, in 2002) and receiving yards (1,368, in 1998). But Diggs has been far less dynamic since the first month. 

Rookie Gabriel Davis, who was very good early and had three 20-plus yard catches, has just two grabs for 18 yards combined over the last three games.

John Brown’s injury — he has a total of five catches for 63 yards since Week 2 — has certainly been a factor. Opposing defenses have paid more attention to Diggs, creating opportunities for veteran slot man Cole Beasley, who has 41 receptions and is on pace for a career high. 

Granted, Allen is still fourth in the NFL in passing yards with 2,172. But he’s down to 11th in yards per pass attempt and 13th in completion percentage. He’s 10th in the league in QB rating at 102.4, so overall his half-season represents a significant step forward.

But at some point, Allen has to stop living off the first month of the season. We’re heading into the real meat of the Bills’ schedule now. The next five opponents have a combined record of 24-12, starting with Seattle at home next Sunday. Then it’s at Arizona, the bye, Chargers, at Niners, Steelers. 

The Seahawks and Cardinals have two of the most potent offenses in the game, as their 37-34 overtime battle a week ago (an Arizona win) will attest. Allen will likely have to approach his September level for the Bills to keep up.

Seattle will be a huge test for a struggling Bills defense next week. This isn’t the Jets or Pats. The Seahawks are averaging 34.3 points a game, No. 1 in the league. Russell Wilson, the current favorite for MVP, has 26 touchdown passes and is on pace to break Peyton Manning’s record of 55, set in 2013.

The Seahawks also allow teams to move the ball at a dizzying pace. They’re giving up 461 yards a game of offense, last in the league. They give up 28.4 points a contest, and it would be worse if they weren’t tied for the league lead in takeaways with 14.

So we could be treated to a shootout at the stadium on Sunday. If Allen can limit turnovers and start making more big plays in the passing game — and if the running game continues to prosper — the Bills offense should get back to the 30-point level they established over the first four games. 

Of course, there was talk of a shootout when the Bills played the Chiefs a few weeks ago, and that never materialized. The Chiefs ran the ball at will and Allen didn’t hold up his end of the bargain. 

The Seahawks are allowing an astonishing 359 yards a game through the air through seven games. If Allen and the passing game can’t come alive in this game, it’ll really be time to wonder.