The Cincinnati Bengals have stumbled a bit out of the gate, starting off 3-0 before falling to 3-2-1 after this past week's sound defeat at the hands of the Indianapolis Colts. So what went wrong for Marvin Lewis' team following their Week 4 bye?

It's easy to point to the loss of A.J. Green, who has only seen 8 targets since Week 2 as he recovers from an injury. The former Georgia standout is one of the league's best receivers, and missing him (along with Marvin Jones, who has yet to suit up this season due to injuries of his own) removes a lot of talent from the equation at wide receiver.

But a quick look at the numbers reveals an entirely different story about why the Bengals have slipped down the power rankings and into the middle of the pack in the AFC.

Getting By Without Green

Last season, the Cincinnati Bengals averaged 3.7 Passing Net Expected Points per game as a team, when adjusted for strength of schedule. That was good enough for 11th in the league, which is a fine ranking for an offense with a superstar at wideout, a decent quarterback, and not a lot else to write home about.

This season, in games during which A.J. Green saw at least one target, the Bengals were averaging 3.9 Net Expected Points per game. That's on par with last year, and perfectly acceptable considering the similarity of last year's Cincy roster when compared to this year's.

But in the three games during which the Bengals have been without Green (the games where he has not been targeted with a pass), the passing offense has actually been slightly better, posting just over five Net Expected Points per game through the air. Obviously there's "small sample size" warning signs flashing all around this statistic, but I think the general conclusion here is easy to see.

This year's Cincinnati offense is a lot like last year's, as the passing attack and running game are both performing similarly to last season. On a per-play basis, the Bengals are actually slightly improved this year as compared to last year when it comes to both throwing and running the ball. With or without Green, the Bengals have been just fine when they have the ball in 2014.

So why are they not running away with the AFC North?

A.J. Green Doesn't Play Defense

The real issue for the Bengals has been their defense, which has fallen off a cliff after a strong start. The Bengals opened up with strong defensive performances against the Ravens, Falcons, and Titans (Green did not play in the game against the Falcons) but then were blasted by the New England Patriots and allowed 37 points in a tie with the Panthers. Last week's 27-0 loss to the Colts was a third-straight weak defensive performance from a team that should be relying on that unit to win games.

In fact, the defense is on pace to finish 160 Net Expected Points shy of last year's team, which finished second in the NFL with a -61.29 Defensive NEP when adjusted for schedule strength. This year's crew is currently on the wrong side of zero (negative NEP is good for a defense, as it indicates their actions reduce the likelihood of scoring for the offense) and trending in the wrong direction after three straight poor performances.

The offense is nearly identical to last season, with the only difference being a six plays-per-game drop in total plays run. But that isn't enough to explain a sharp decline in defensive efficiency from one of the league's best units a year ago.

The team is currently 23rd in the NFL in sacks and struggles to pressure quarterbacks consistently, which many fans will blame on the departure of Michael Johnson to Tampa Bay. He didn't post impressive sack numbers in 2013, but he did get after opposing quarterbacks regularly and commanded the respect of opposing offenses. That sort of player simply isn't there this year along the Bengal defensive line, and that may be the reason for their struggles.

The defensive downfall could also be attributed to the loss of Mike Zimmer, whose coaching efforts with the Cincy defense earned a head coaching gig with the Vikings this past offseason. Was it possible that Zimmer's defensive genius was enough to elevate an average group of players to new heights as one of the best units in the league?

Either way, it's clear that A.J. Green's absence isn't the issue for the Bengals this season. They'll welcome him back with open arms, but only because he'll, hopefully, help them score more points to overcome a defense that's playing significantly worse than it did a year ago.

Marvin Lewis will have a lot of explaining to do if his Bengals are unable to make this playoffs this year because of their defense, which is supposed to be his strong suit as a veteran defensive coach with a Super Bowl ring to show for his efforts. With a defense that ranks 21st overall through Week 7, Lewis needs to right the ship soon if the Bengals hope to realize their full potential in 2014 and make a serious run at the Super Bowl.