Arizona has the NFL's best defense when it comes to covering wide receivers, but the quality is not spread equally across the secondary. Patrick Peterson is outstanding, and Tyrann Mathieu, when he was healthy, was playing very well at nickelback. Jerraud Powers is another story. According to our game-charting data through Week 16, Powers has given up 9.5 yards per pass, with a 51 percent success rate. Peterson, by comparison, has given up 6.4 yards per pass with a 60 percent success rate. (Success rate for cornerbacks is explained here.) Powers has been targeted more often than Peterson, even though Peterson is usually covering the opponent's top receiver.
Arizona also has trouble covering tight ends, where the Cardinals rank 20th in DVOA. When San Francisco beat the Cardinals back in Week 6, Vernon Davis torched the Arizona secondary for eight catches for 180 yards and two touchdowns. And who was covering Davis on three of those plays, including a 61-yard touchdown? Powers. Peterson covered Anquan Boldin in the first game, but he'll probably be on Michael Crabtree this week. If Powers is covering Boldin, that makes both Boldin and Davis enticing targets for Colin Kaepernick.
One other problem for the Cardinals is that no defense blitzes more than Arizona, yet very few quarterbacks are better against the blitz than Kaepernick. His 85.1 QBR against the blitz is the highest in the league for a quarterback with at least 10 starts.
While the Packers are likely to have a much improved offense, the Bears will run the same defense that has ranked 31st in defensive DVOA over the second half of the season. We all know Chicago has been dismal against the run; the Bears have allowed 5.4 yards per carry. No other defense is above 4.8. Things aren't much better against the pass. The pass rush, for example, is just 25th in Adjusted Sack Rate.
What about the secondary? Bears left cornerback Tim Jennings may have had an excellent 2012, but this season, game-charting data through Week 16 has Jennings giving up 9.6 yards per pass, with a success rate of 46 percent -- about 10 percentage points below the average cornerback. The Bears also rank 26th in DVOA against tight ends, giving up a league-leading 74 yards per game. In the first Green Bay-Chicago game, Andrew Quarless actually led the Packers with five receptions.