Ann Heisenfelt/Associated Press

The Minnesota Vikings were a popular pick to make the playoffs this season.

With Adrian Peterson returning, Teddy Bridgewater developing and head coach Mike Zimmer further pushing his identity onto the team's defense, there were few reasons to be concerned about the Vikings. The biggest threat to their success was the team's porous offensive line.

Porous offensive lines are common across the league this season. As teams look to become more and more pass-inclined, there is a greater demand for pass-protecting linemen than are available.

Of all the offensive lines in the league, the Vikings have the worst. The unit was among the least talented in the league entering the season and have only gotten worse since that point. Injuries to starting center John Sullivan and starting right tackle Phil Loadholt robbed the unit of its two best players.

Sullivan should eventually return—he is on short-term IR—but Loadholt has been lost for the season with a torn Achilles.

Neither player was an All-Pro talent, but both were dramatically better than their available replacements. Rookie T.J. Clemmings has started for Loadholt at right tackle, while 33-year-old veteran Joe Berger has been starting at center.

Clemmings is a physically gifted player who fell to the fourth round of the draft because of health question marks. His athleticism hasn't shone through on the field as he has been repeatedly exposed in pass protection.

Berger is on the opposite end of the spectrum. He has played 11 seasons in the NFL, starting just 42 games. He is not a technically sound or consistent player, nor does he offer the athleticism to make impact blocks or redirect defensive linemen in pass protection.

The Vikings offensive line is a weakness that can be overcome by quality quarterback play and the running ability of Adrian Peterson. However, that's extremely difficult when facing more talented defensive fronts.

In Week 4 against the Denver Broncos, that was made clear.

Having overcome the Detroit Lions and San Diego Chargers in consecutive weeks, the Broncos offered the Vikings the toughest test that they had faced all season. While it's possible that the Broncos will be the best defense the Vikings face all year, the extent of their struggles are concerning for other matchups.

The Broncos sacked Bridgewater seven times, with an eighth sack that was negated by a holding penalty in the secondary. During the final minutes of the fourth quarter, when the offense was attempting to drive for a game-winning score, the Broncos sacked Bridgewater and forced a fumble that sealed their victory.

Defensive coordinator Wade Phillips was able to get to Bridgewater by blitzing him, but the quarterback also created a number of big plays when he sent extra defenders.

Beating blitzes shouldn't be a concern for the Vikings. Bridgewater may have limited support, but his acumen and passing ability will allow him to be effective against the blitz. Pro Football Focus grades him at plus-0.9 for his career against the blitz.

To put that in context, fellow 2014 quarterbacks Derek Carr (minus-11.8) and Blake Bortles (minus-14.9) grade dramatically worse than Bridgewater over their careers so far.

The bigger concern for the Vikings is their inability to hold up against four-man rushes. Kyle Rudolph and Mike Wallace are the team's two best receiving options. Neither is a matchup problem for opposing defenses or someone who can consistently win contested catches.

Without a matchup threat to draw coverage or a receiver to win contested catches, opposing defenses can sit in flooded coverages behind their four-man rushes and squeeze all of the space away.

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The above image comes from a play against the Broncos. At the time the image is frozen, Bridgewater is hitting the top of his drop. He has space in the pocket at this very moment because his protection has six available blockers to repel four pass-rushers.

While Bridgewater has space to release the ball at this point, he doesn't have any outlet.

He only has four receivers running routes, and three of those have released vertically downfield. One is bending his route infield at this point of the play, but there are multiple defenders there to cover his movement.

Having seven defenders in coverage against just four receivers means that Bridgewater has to hold onto the ball. If he was to release the ball at this point of the play, he would be forcing it into a spot where the defense had as much, if not more of a chance to intercept it as his receiver would have to catch it.

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Bridgewater's pocket doesn't stay intact for long. He has the space to release the ball, but even with his ability to throw with anticipation and lead receivers to space, there isn't a viable outlet for him to get rid of the ball at this point of the play.

He is forced to step up to try to avoid one defender penetrating the pocket from the middle, but another is there to prevent him from looking downfield.

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The Vikings doubled Von Miller and DeMarcus Ware on the edges, leaving the Vikings interior linemen to handle the less talented pass-rushers on the Broncos defense. The interior of the line was failing quickly, but Bridgewater's deeper drop aided them.

If the quarterback had a quick option on the play, he would have been able to find it.

He didn't have a quick option, so the two defensive linemen were able to trap him with a pincer movement in the pocket. He had no viable escape route in any direction, so he tried to skip through the tiny gap that had developed between his two teammates.

Whenever you are evaluating a quarterback, you have to ask what he should have done or what he could realistically have done. On this play, Bridgewater realistically had no chance.

Three of Bridgewater's seven sacks against the Broncos came on four-man rushes. The other four came on five-man rushes when similar situations developed. The Broncos were isolating the Broncos offensive linemen in one-on-one situations to take advantage of their lack of talent.

Sacks weren't the only problem though. The limiting effect that a poor offensive line has on a passing game can be huge.

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On this play, the Broncos again only rush four defenders after the quarterback. The Vikings are running vertical routes outside with a shallow crossing route that draws the attention of the two linebackers underneath.

The above image captures the moment that Bridgewater hits the top of his drop.

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Left tackle Matt Kalil was beaten immediately on this play. Kalil could only lean into the edge-rusher to slow him down as he penetrated the pocket. His effort out of desperation gave Bridgewater the opportunity to adjust and slide away from the pressure.

Bridgewater takes that opportunity, but he is faced with a second defender who is unblocked.

That defender stunted around Ware to the outside. Both of the interior linemen stood still in the middle of the field before turning to face each other as Bridgewater was being pressured in his own end zone.

Ultimately, the quarterback was forced to throw the ball at the feet of his tight end, Kyle Rudolph, just to avoid a safety.

While it didn't matter for this play specifically, it's telling that the Vikings failed to pick up the stunt on the other side of the offensive line also. Even if Bridgewater had somehow managed to evade the first two waves of rushers, the third would have gotten to him.

Moving the ball in these kinds of situations is extremely difficult and requires the quarterback to make exceptional plays on a consistent basis. Bridgewater can make these plays, but it's a lot to ask of any quarterback to do so as often as they need him to.

That's even more true when you consider how little Bridgewater has played at this level.

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On this occasion, Wade Phillips doesn't blitz, but he disguises his coverage by alignment at the snap. He presses each of his three linebackers between the tackles so the offensive line can't identify who is coming and who is dropping back into coverage.

At the snap, the three linebackers drop into coverage, leaving the team's four rushers to chase the quarterback.

Even though Phillips didn't blitz, his disguise had fooled the Vikings protection. Von Miller was left alone in space against a running back. The back willingly got in front of Miller, but he could only slow him down for a moment.

Had one of the three linemen who focused on one defender in the middle been more active, they would have had an opportunity to get back and cut Miller off. Instead, the edge rusher was able to easily bounce the back off of him and close in on the quarterback.

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As Miller gets past the back, Bridgewater feels the incoming pressure and keeps his eyes downfield. Bridgewater is able to escape Miller initially, but he doesn't have space to run to and Miller is a much better athlete than him.

Bridgewater releases the ball as he is falling to the ground with Miller around his waist.

The Broncos were playing Cover 3 on the back end of this play. Bradley Roby had made an initial mistake in his coverage, allowing Stefon Diggs to get a step on him. T.J. Ward, the deep safety on the play, didn't have an opportunity to work across the field to cut him off.

Had Bridgewater been given time in the pocket here, he would have had an opportunity to throw a 48-yard touchdown pass.

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Instead of getting time to lead Diggs downfield, Bridgewater has to do exceptionally well to just give Diggs an opportunity to make a play on the ball 20 yards downfield. Diggs helps his quarterback out by adjusting to the ball before making a contested catch.

Diggs and Bridgewater had to do a huge amount of work to simply gain half of the yards the defense should have given up.

It's this inability to take full advantage of opportunities that can be traced back to the limitations of the team's pass protection. On this occasion it wasn't the offensive line's full responsibility, but a more active, aware group of individual blockers would have recovered to give the running back more help against Miller.

This kind of play is a great example of how the better defenses are going to pile the pressure on the Vikings offense through scheme and talent mismatches.

The Vikings don't have an easy schedule when it comes to pass-rushing matchups either. They still have to face the Kansas City Chiefs, St. Louis Rams, Oakland Raiders, Green Bay Packers, Atlanta Falcons and Seattle Seahawks. Each of those teams have multiple high-quality pass-rushers who will be mismatches for the Vikings offensive line.

Standing at 2-2, the Vikings are theoretically in a good position to make it to the playoffs this season. If they do manage it, it's going to be a much greater feat than initially expected.