Gauging the value of a defensive player can be difficult if you stick with traditional statistics – tackles, sacks, interceptions, etc.  For example, is a linebacker that gets 10 tackles per game really good if most of those tackles come on 5 yard runs and the opposing offense averages 5.5 ypc?  If a DE gets 10 sacks a year, but the opposing QBs complete 64% of his passes for an average of 280 yards per game were those 10 sacks really that valuable?

In order to evaluate how good an individual defensive player is AccuScore likes to look at how the opposing offenses perform in 3 situations:

1. vs. the defense when the player is active
2. vs. the rest of the league’s defenses
3. vs. the defense when the player is inactive

Now if you look at just #1 and #2 a player who sees 1 snap a game would have the same statistics as a player involved with 60 snaps.  It is easier to gauge an individual defensive player’s value when he actually misses a significant number of games.

An accompanying article describes how much value the Giants’ Michael Strahan provides in simulations vs the Patriots.  For the Patriots I looked at a number of defensive starters.  Defensive standouts like Asante Samuel, Tedy Bruschi and Mike Vrabel weren’t included in this analysis because they have missed just 2 games between them in the past 2 seasons.

When looking at players like Junior Seau, Rodney Harrison, Vince Wilfork, Rosevelt Colvin and Richard Seymour it became clear that the one of the most valuable individual defensive players on the Patriots is Junior Seau.

On January 19th, Seau turned 38 years old, a full 10-12 years past his physical prime.  Yet against the Chargers it was Seau who was blasting into the backfield to tackle Michael Turner for a loss on 3rd and short in the Red Zone.

With the exception of the Patriots’ Sack rate (high 7.9% with Seau, even higher 9.0% without) and completion percentage (impressive 58.5% with Seau, just 55.6% without Seau) all other key stats are considerably better with Seau active.  For example, the Patriots were an average defense in rushing yards allowed per game without Seau, but considerably better with him.  Obviously he is not a defensive back, but the defense still was much better limiting long pass completions when Seau was in the lineup (6.2% vs. 9.1%). 

We made conservative assumptions in the simulations and removed Junior Seau from the team to see what would happen to the Patriots’ winning percentage.  In simulations where Junior Seau did not play the Giants chances improved from 25% to 28% (+3 percentage points, a 12% increase in winning percentage). 

Now 3 percentage points may not sound like a lot, but in terms of simulation point spread the Patriots go from being a -10 favorite to a -7 favorite.  The Patriots have won 3 Super Bowls, each by 3 points.  If Junior Seau provides a 2-3 point impact, given the Patriots’ history, that could be the difference between winning and losing.