Top Week 6 fantasy matchups

Arian Foster has been a fantasy stud this season since he returned from injury. AP Photo/Eric Gay

The first five weeks of the NFL season have seen several surprises on defense, some pleasant and some not so pleasant. For instance, as has come as a shock to this 49ers fan, San Francisco's pass defense hasn't been horrible. In contrast, the Packers' pass defense has been worse than most expected. The question is, can we trust what we've seen and number crunched after little more than a month of the NFL season?

To put it as succinctly as possible, yes. A couple of weeks ago, we posted a chart on the Football Outsiders website illustrating the point in the season at which our play-by-play efficiency statistic, DVOA, becomes a reliable indicator of teams' true quality. For defenses, that point is five weeks into the season.

For fantasy owners, that's a good thing to know. It means that they can start playing matchups more confidently with respect to position players. It means that they can jettison underwhelming fantasy defenses without worrying that a five-sack, three-interception shutout with two kick-return touchdowns is just around the corner.

In that vein, here are several defensive units -- either surprisingly good or surprisingly bad through five weeks -- that you can confidently assume will continue to be so for the rest of the season:

Washington Redskins pass defense (2010: 27th, 2011: third)
It's amazing what a pass rush can do for a pass defense. Last season, the Redskins had the 29th-ranked pass rush, according to our metrics. This season, it's the best in the NFL and has actually gotten better since I cited it three weeks ago as a catalyst behind Washington's 3-0 start.

Jacksonville Jaguars run defense (2010: 28th, 2011: 10th)
It's amazing what signing young, already-successful players at middle linebacker and strong safety can do for a run defense. Where the signings of Paul Posluszny and Dawan Landry seem to have helped the most is in preventing opposing running backs from big gains. According to our metrics, the Jaguars have improved from 32nd to 10th in open-field yards allowed per carry, which only counts yardage gained more than 10 yards beyond the line of scrimmage; this despite having already faced dynamic runners such as Chris Johnson and Darren Sproles.

Chicago Bears pass defense (2010: fifth, 2011: 24th)
To hear Chicago's coaches and players tell it, the cause of this decline is a simple lack of execution. Our take: Musical chairs at safety hasn't helped, age is starting to catch up with them and they've faced five of the best quarterbacks in the league so far. The problem going forward is that the first two things aren't going to change, and the next four games after this week are against Josh Freeman, Michael Vick, Matthew Stafford and Philip Rivers.