The post-Brady Patriots are 3-1 heading into their showdown with the San Diego not-so-super Chargers (2-3) Sunday night. Some New England fans might even feel pretty good about the team’s prospects this season in the aftermath of the Tom Brady injury.

But the Patriots right now have more issues than Lindsay Lohan – issues that would cost the team dearly this season, even if a certain future Hall of Famer was still behind center.

Take, for example, the issue of New England’s pass defense this season, which has more holes in it than Sonny Corleone’s corpse.

And porous pass defenses always spell trouble in the NFL. Shut-down pass defenses always spell success.

Just look at Bill Belichick’s dynastic Patriots of the 21st century: They won Super Bowls when they dominated on pass defense. They struggled when they played poor pass defense.

And right now, the 2008 Patriots are struggling to play pass defense.

The Cold, Hard Football Facts judge pass defenses not by yards allowed – which is virtually meaningless – but by defensive passer rating. In other words, we take the formula used to rate quarterbacks and apply it to pass defenses.

Defensive passer rating is what we call a “Quality Stat” because it has a direct correlation to winning football games. Play great pass defense and you win games. Play poor pass defense and you lose games.

Just look at Bill Belichick’s Patriots when they played great pass defense and when they played poor pass defense.

The 2001 Patriots fielded one of the best pass defenses in football. They allowed 15 TD passes, picked off 22, and posted a defensive passer rating of 66.7 (80 is about average). The 2001 Patriots won the Super Bowl.

The 2003 Patriots dominated on pass defense. They allowed 11 TD passes, picked off 29 and led the league with a stifling 56.2 defensive passer rating. The 2003 Patriots won the Super Bowl.

The 2004 Patriots fielded one of the league’s best pass defenses. They allowed 18 TD passes, intercepted 20, and posted a defensive passer rating of 75.3. Coupled with a strong offense, the 2004 Patriots won the Super Bowl.

The worst teams of the Brady-Belichick Era, meanwhile, fielded the worst pass defenses:

The 2002 Patriots struggled on pass defense, with a 78.2 defensive passer rating. The 2002 Patriots went 9-7 and missed the playoffs.

The 2005 Patriots also struggled on pass defense, with an awful 87.8 defensive passer rating. The 2005 Patriots went 10-6 and were bounced from the playoffs in the divisional round.

Fast forward to 2008, and you find a team that plays pass defense more like the struggling teams of 2002 and 2005 than it does the champions of 2001 and 2003.

Four games into the 2008 season and New England’s defensive passer rating is 84.7, in the bottom half of the league and among the worst Belichick has put on the field during his tenure with the Patriots.

If you take into consideration the quality of the teams and the offenses that the Patriots have faced in the first quarter of 2008, it’s a pretty sad state of affairs. After all, not one of New England’s first four opponents has a winning record.

And, with the exception of future Hall of Famer Brett Favre (an aging player with a new team), the quarterbacks the Patriots have faced reads like a forgettable who’s who of NFL journeymen: Brodie Croyle, Damon Huard, Chad Pennington and even the immortal J.T. O’Sullivan.

Yet even against this collection of castoffs, New England’s pass defense is still in the bottom half of the league. And it’s only going to get tougher: three of New England’s next four opponents are Phillip Rivers, Jay Cutler and Peyton Manning – a much stronger collection of quarterbacking talent than the Patriots have faced in their first four games.

New England fans must come to grips with the fact that Tom Brady’s Patriots might struggle to win with this kind of sub-par pass defense. In fact, Brady’s Patriots have struggled to win this kind of pass defense.

It’s unreasonable to ask for anything better than a struggle from Matt Cassel’s Patriots.

Kerry J. Byrne is the publisher of ColdHardFootballFacts.com .  His self-congratulatory column will appear here each Wednesday during football season. Send fawning praise, death threats or pictures of your 19-year-old sister to contact@coldhardfootballfacts.com.