Until two weeks ago, St. Louis quarterback Marc Bulger was enjoying perhaps the finest season of his career. Through the first eight and a half games, Bulger completed 65% of his passes and averaged 7.9 yards a pass attempt, both among the top figures in the league. He had 13 touchdowns and had thrown just one interception all year.

Then, in the second quarter against Seattle, Rams left tackle Orlando Pace tore a triceps muscle and was forced out of the game. The Rams went on to lose that game on a last-second field goal, and this week the Rams were shut out by Carolina 15–0.

In the six quarters since Pace was injured, Bulger is completing 57% of his passes and averaging just 4.4 yards a pass attempt. He has no touchdowns and twice as many interceptions as he had the entire rest of the season.

The Rams are just one of many teams struggling this season because of injuries on the offensive line. Quarterbacks and running backs may be considered the “skill players” on an NFL offense, but this year provides plenty of evidence that the most important offensive skill may be blocking.

Very few teams have a single lineman as important as Pace, but the Seattle Seahawks offer a good example of how multiple offensive line injuries can affect a team. Star left tackle Walter Jones hasn’t missed any time, but he’s been less effective thanks to an ankle sprain suffered in the first game of the season. Left guard Pork Chop Womack has missed five games, right tackle Sean Locklear has missed four, and center Robbie Tobeck has missed two.

All of this upheaval along the offensive line has caused one of the NFL’s best running games to completely disintegrate. The injury to reigning league MVP Shaun Alexander gets most of the headlines, but we can isolate the issues along the line with Adjusted Line Yards, a metric that cuts off long runs to pay closer attention to what happens close to the line of scrimmage. Last year, the Seahawks ranked sixth in the league in Adjusted Line Yards, and this year they are dead last.

The offensive line injuries are affecting the passing game as well; Seattle quarterbacks have already been sacked 34 times through 10 games after taking just 27 sacks all of last year. Pass pressure is also the reason that word is “quarterbacks” instead of just “quarterback.” If he had been guarded by a healthy offensive line, perhaps starting quarterback Matt Hasselbeck would not have suffered a torn MCL against Minnesota, leaving the Seahawks to start inferior backup Seneca Wallace for six games.

The Seahawks have managed to go 6–4 this season despite their injury issues, and still lead their division. The same can’t be said for the Cincinnati Bengals. Expected by many to be Super Bowl contenders this season, the Bengals are just 5–5 and three games behind Baltimore in the AFC North. Two big reasons: starting center Rich Braham has missed eight games, and starting left tackle Levi Jones has missed six.

The Cincinnati offense struggled at midseason as replacements Andrew Whitworth and Eric Ghiaciuc gained NFL experience. Quarterback Carson Palmer straightened out his fundamentals as he recovered from off-season knee surgery, but it was hard to tell because he was under constant pass pressure. Running back Rudi Johnson was held under 70 yards four times between Week 4 and Week 8; last year, he only failed to clear to clear 70 yards twice.

The Bengals offense has turned things around now that the replacements have experience, and scored 41 and 31 points over the past two games. When you replace an injured lineman with a promising youngster, which both Cincinnati and Seattle did, you at least know that some improvement is likely over time.

But what if you replace your most important lineman with an over-the-hill veteran who is already surviving on his knowledge of technique alone and can’t really get any better?

That’s the question that faces the Giants as they try to regain their momentum and win the NFC East. It’s not a coincidence that the Giants have lost two straight games without left tackle Luke Petitgout, who was lost for the season when he broke his leg in the first quarter of the Giants-Bears battle two weeks ago.

Petitgout’s replacement, Bob Whitfield, is 35 years old and has been in the league since 1992. He has started just four games over the past three seasons. For two weeks he hasn’t been able to do the job, and quarterback Eli Manning has been running for his life. The Jaguars only sacked Manning once on Monday night, but that was only because he threw the ball away so many times. Tiki Barber caught just one pass because he was constantly held back to help with blocking.

Just as Bulger’s numbers turned south without Pace to protect him, Manning has had two of the worst games of his career without Petitgout. The Rams have dropped out of playoff contention, and if the Giants can’t fill that hole at left tackle soon, the same will happen to them.

Mr. Schatz is the editor in chief of FootballOutsiders.com.