Every year, about half of the 12 playoff teams change over. And every year when we predict the results at the beginning of the season, we have trouble understanding how there could possibly be so much turnover in this league.

But only four weeks into the 2011 season, we’re already beginning to get the sense that more than half a dozen 2010 playoff teams are in serious trouble. Let’s take a look at all 12 of the franchises that played January football last year:

AFC

1. New England Patriots: Once again on track to earn a first-round bye with a 3-1 start.

2. Pittsburgh Steelers: Only 2-2 out of the gate. The run defense has been awful, they aren’t forcing turnovers (one through four weeks), they aren’t protecting the quarterback (again), and now Ben Roethlisberger’s hurt. Technically, they’re tied for last place in the AFC North.

3. Indianapolis Colts: Without Peyton Manning, they’re 0-3 and on the verge of falling to 0-4 on Monday Night Football.

4. Kansas City Chiefs: Ravaged by injuries, they won their first game of the season Sunday against Minnesota. The season is probably lost.

5. Baltimore Ravens: The 3-1 Ravens are in good shape following a Sunday Night Football victory over the Jets.

6. New York Jets: They’re 2-2 and in trouble. The offensive line is in shambles without Nick Mangold and the run defense has disappeared. Today, they lost reliable linebacker Bryan Thomas for the season.

NFC

1. Atlanta Falcons: In serious jeopardy of dropping to 2-3 when they host Green Bay next Sunday night. And it could be worse: they nearly lost to the Seahawks in Week 4. The offensive line has been a disaster.

2. Chicago Bears: Another team lucky to be 2-2. Jay Cutler’s getting crushed. They barely beat Carolina at home this week.

3. Philadelphia Eagles: We’ve gone on and on about the Dream Team’s nightmare start. And now the injuries are piling up with Trent Cole and Jason Peters going down.

4. Seattle Seahawks: They’re 1-3 and in a 2.5-game hole already in the terrible NFC West. Seattle was only 7-9 last year, so the expectations weren’t there anyway.

5. New Orleans Saints: They’ve been solid with three wins in their first four games. And the killer offense should only get better as they get healthier.

6. Green Bay Packers: A perfect 4-0 start. It’s not even an argument: this is the best team in football.

So eight teams — Pittsburgh, Indy, KC, the Jets, Atlanta, Chicago, Philly and Seattle — are in serious trouble already, with teams like Detroit, San Diego, Houston, Buffalo, Washington, the Giants and Tampa Bay looking to steal their spots.

It’s amazing how large a role injuries play in deciding who goes to the playoffs each year. Just today, the Jets and Eagles were rattled by demoralizing injuries, and Pittsburgh saw Roethlisberger get hurt on Sunday. And no team has been crushed harder by big injuries than the Chiefs have.

Hate to say it, but luck factors into what goes down every single year in this league.