<blockquote>Penetration from front 4 is essential to any variation of the Cover 2. If you are forced to blitz, like the Bears were last year, you just can't run the defense. You need those 4 down lineman to penetrate and disrupt running plays or to pressure the QB on passing plays, so your remaining 7 defenders can play their zones accordingly. This is why Lovie calls the signing of Defensive Line Coach Rod Marinelli such an important piece of the puzzle. If Marinelli can max out the potential of the D-Line the defense will get back to more Cover 2, and subsequently less blitzing. Last years numerous blitzes and Mug look (lining the LB's up close to the line of scrimmage near the A gaps) took away from what Lovie Smith had built the defense up to be.</blockquote>
So after reading wiltfongjr's excellent piece on the cover 2, I quickly realized the numbers back up and expand upon one of his main points, using the 2008 Bears as an example. Namely, pressure on the passer is a key to the success of the cover 2 defense.
If you look at the breakdown they have at footballoutsiders.com, you'll see that the Bears had the 7th-best D in the league, according to both their DVOA stat (5.5% better than average, see here for an explanation of how FO arrives at these numbers) and their weighted number (2.5% better than average). Their rush D was particularly good -- 16.5% better than the average team -- and was the 5th-best in the league. Their weakness was against the pass, where they were 3.3% worse than average, and only ranked 11th in the NFL.
Now, if you look at the numbers they had against different types of receivers, they actually did quite well. They did better than average against #1 WR's, #2 WR's, "other WR's" (i.e. 3/4/5), and TE's. They only were worse than average against RB's (where they were pretty bad). This may lead you to ask how they were below average against the pass, if they were good against everything but RB's, who don't get many passes thrown their way. The answer is simple, and right on the page those numbers come from: DVOA of defense against receivers does not include sacks or passes with no intended receiver listed. To put it another way, the problem was the Bears simply allowed too many passing plays to turn into pass attempts; they didn't get enough sacks. The numbers also bear this out as the Bears were horrendous at rushing the passer. They had a sack rate of, 5%, which was 6th worst rate in the league. This was the case despite the Bears high blitz rate. This blitzing was also probably part of the reason the RB was successful catching passes out of the backfield, as it was often the OLB's blitzing, who would otherwise be responsible for wrapping up the RB after the catch.
If you ask me, the Bears need to upgrade their pass rush this offseason more than anything else on D. When you blitz the passer and don't get pressure on the QB, you're going to have trouble in coverage no matter your scheme; that's not on the secondary, it's on the D-line. And if this happens when you're playing the cover-2, the problem is accentuated. The passing D was a problem last season, but it wasn't the secondary or "coverage" that was the issue. Rather, the problem was the D-line and the pressure (or lack thereof) on the QB. They didn't really make any big acquisitions in terms of pass-rushing D-linemen this offseason. Hopefully, Rod Marinelli can turn improve the talent they already had.