The Seattle Seahawks' offense hasn't lived up to expectations this season with the addition of TE Jimmy Graham. Instead of taking a step forward, Russell Wilson & Co have stumbled, falling from 11th last season in offensive points scored per game to 26th in 2015. Part of the explanation has to do with what the Seahawks gave up to get Graham last offseason: all-pro center Max Unger.

Before we delve into Unger's importance, let's touch put Seattle's offensive front in the proper context. Since Wilson was drafted in 2012, the Seahawks have declined each season in our Pass Protection metric (read more about it here). After winning the battle upfront on 49.8 percent of pass plays in 2012, that number fell to 47.2 percent in 2013 and 43.3 last season. So far this season, it's 41.3 percent, which is not only last in the league, but dead-last for any offensive line in the past four seasons. The severity of the Seahawks' O-line problems have been masked by Wilson's mobility and the infrequency with which they pass.

The numbers being as they are, you have to wonder how much worse off Seattle would've been without Unger from 2012-14. To calculate the full effect of Unger's performance on the team's overall offense, I calculated a WOWY (With Or Without You) stat for the Seattle offensive line from 2012-15. Using the Expected Points Added metric -- which is a measure of the impact of each play outcome based on the down, distance and yard-line at the snap -- we can calculate how the Seattle offense fared with each offensive lineman on or off the field. For example, over the last four seasons, when guard James Carpenter was on the field, the Seahawks averaged 0.07 EPA per play, compared to 0.09 EPA per play when he wasn't on the field. The difference (minus-0.02) suggests Carpenter may have been a slight downgrade from other options at his position.

Here's the full breakdown of Seattle's offensive linemen since 2012:

EPA/Play With/Without SEA O-linemen On Field* Player, Position Snaps With Without Diff Max Unger, C 2015 0.11 0.04 0.07 Breno Giacomini, T 1472 0.09 0.08 0.01 Paul McQuistan, G 1692 0.1 0.09 0.01 Alvin Bailey, T 573 0.07 0.08 -0.01 Justin Britt, G 1581 0.08 0.09 -0.01 Lemuel Jeanpierre, C 513 0.07 0.08 -0.01 Russell Okung, T 2621 0.08 0.09 -0.01 James Carpenter, G 1847 0.07 0.09 -0.02 J.R. Sweezy, G 2766 0.08 0.1 -0.02 Michael Bowie, T 505 0.06 0.09 -0.03 Drew Nowak, C 448 0.02 0.09 -0.07 Patrick Lewis, C 376 0.01 0.09 -0.08 Garry Gilliam, T 618 0.01 0.1 -0.09 *From 2012-15

Unger's numbers are remarkable. When he was on the field, Seattle averaged 0.11 EPA per play, the most of any Seattle lineman in the Wilson era. And when he wasn't on the field, the Seahawks' EPA per play fell to 0.04. Those might sound like tiny numbers, but the difference of 0.07 EPA over an average of 64 snaps per game adds up to 4.5 points. That's enough to take the Seattle offense from 20th overall in EPA per play, where they currently rank, to 12th.

Because Unger's departure occurred over the offseason, when various changes might affect an offense, his apparent value could be exaggerated by other offseason factors. So I looked at his WOWY numbers from just the 2014 regular season, when he started six games. Turns out, doing that only made him look more valuable -- the Seahawks averaged 0.16 EPA per play with him on the field in 2014, compared to 0.04 EPA per play without him, a difference larger than over the entire period.

This method is a fairly rough approximation of value, because it does not account for the other players who happen to be on the field simultaneously, including the opposing defense. And it doesn't work well for players who have been fixtures at their positions, like J.R. Sweezy at guard or Russell Okung at tackle. But Unger's difference is so large and stretches over such a long period of time that his absence has undoubtedly been a principal cause of the Seahawks' struggles this season.