History lesson: Losing Strahan going to hurt Giants

In honor of Jonathan Ogden's retirement, I recently took a look at what happens to teams that lose a top offensive lineman. Of course, Ogden is not the only sure Hall of Famer who announced his retirement this month. Giants defensive end Michael Strahan, fifth all-time with 141.5 sacks, is also walking off into the sunset, after finally getting a championship ring from New York's Super Bowl XLII upset of the Patriots.

It's rare for an offensive lineman of Ogden's caliber to retire while he's still making Pro Bowls, but it is rarer still for a pass-rusher of Strahan's quality to retire in similar fashion. Strahan had nine sacks last year at the age of 36, making him only the second player since 2000 to ring up eight or more sacks after age 35. (Bruce Smith was the other.) Strahan is also the first player in this century to retire after a season with seven or more sacks. Nobody had done that since Kevin Greene and Chris Doleman retired after the 1999 season.

While there aren't a lot of star pass-rushers who have retired in their prime, there are plenty who have changed teams, and those players can give us a good idea of what to expect from the post-Strahan Giants. From 1998 to 2007, 46 different players did not return to their teams after a season with at least seven sacks. Forty-two of those players switched teams via trade, free agency or expansion. Greene and Doleman retired, as noted above. So did Reggie White (for one year, at least) after 1998. The final player is Kansas City linebacker Derrick Thomas, who died in a tragic car accident in early 2000.

A couple of teams actually lost two of these players -- leaving a total of 43 teams that lost at least one star pass-rusher who recorded at least seven sacks the season before. Here's how those teams fared the next season, according to both conventional statistics and the Football Outsiders' DVOA ratings (explained here) for both total defense and pass defense.

These teams declined as a group -- although the decline seems fairly small -- but we can see that these teams went from having better-than-average defenses to slightly worse-than-average defenses. (Remember, an average defensive DVOA is 0%, and because DVOA represents scoring, a bad defense allows a higher DVOA, just like a bad defense allows more points and yards.)

The seven-sack minimum does mean some players don't quite qualify as "great pass-rushers." Great pass-rushers don't usually get left unprotected in an expansion draft, like Gary Walker was in 2002. If we only look at teams losing players with at least nine sacks, the effect is a little larger:

Not every great pass-rusher comes from a great pass-rushing team, of course. Some fans may think the Giants defense can easily make up for Strahan's departure because Justin Tuck is ready to start, Osi Umenyiora is still at his peak and Mathias Kiwanuka is once again healthy. However, recent history shows that top pass-rushing defenses like the Giants take the biggest hit when they lose their stars.

Last year, the Giants led the league with 52 sacks. They are one of just 39 teams since 1998 with at least three sacks per game during the regular season. Now, as you might expect, these teams almost all had fewer sacks the next year. It's very rare that a defense plays at that high of a level for two straight seasons. Only five of these teams had at least 48 sacks in consecutive seasons, and only two -- the 1998 and 1999 Rams, and the 1999 and 2000 Titans -- actually had more sacks the following year.

Still, not all these defenses declined equally. The 12 defenses that lost at least one star pass-rusher (seven or more sacks) in the offseason deteriorated a lot more than the 26 teams that did not.

Strangely, the teams that didn't lose a top pass-rusher actually saw a slightly larger decline in wins than the teams that did, even though the latter group clearly lost more on defense.

Does adding a top pass-rusher help as much as losing a top pass-rusher hurts? No, not even close. In fact, top pass-rushers switching teams have an effect roughly opposite that of top offensive linemen switching teams: Offensive linemen tend to have a bigger impact on their new teams, while top pass-rushers tend to have a bigger impact on the teams that lose them. Here's a look at what happened to the 39 teams that added a top pass-rusher from 1998 through 2007 (not counting expansion teams):

A slight improvement on defense and slightly fewer wins -- not exactly what the Cleveland Browns had in mind when they brought in Shaun Rogers and Corey Williams this offseason. (Yes, a guy like Rogers will also do a lot to stuff the run, but we're not analyzing that here.)

Aaron Schatz is president of Football Outsiders Inc. and the lead author of Pro Football Prospectus 2008.