After Chicago hired Lovie Smith after the 2003 season, the first major move the team made in the Lovie era was using their 1st and 2nd round draft picks during the 2004 draft on DTs Tommie Harris and Tank Johnson, respectively. These are not exciting players, but they hold the middle, and occupy blockers that could be going to double team the good pass rushing defensive ends or blocking the linebackers. Chicago has been 29-19 (regular season) since they drafted Harris and Johnson. This has been a part of Chicago's success over the past several seasons. It was no accident that Chicago's surprising 13-3 record in 2001 occurred the same year they signed free agent DTs Keith Traylor and Ted Washington. The defense starts from the inside out.
That is why the release of Tank Johnson is such a big deal. This is a huge blow to Chicago's defense. Chicago's defense isn't just Harris and Johnson, but in 2006 when both played Chicago allowed just under 11 points/game, and allowed just under 23 points/game when one or both of them didn't play. Chicago isn't guaranteed to allow 23 points/game next season, but that statistic indicates that Chicago plays much better when both of their starting defensive tackles were playing.
Johnson backed Jerry Angelo into a corner where he had a tough choice to make. No matter what Angelo did, his decision was going to hurt Chicago on the field. Windy City Gridiron isn't too concerned about the loss of Johnson, but does put a disclaimer on the end that Plan B, aka DT Anthony Adams, better be able to take over for Tank. It would seem unlikely that an undersized defensive tackle with six career sacks, who was just cast off by one of the worst teams in the NFL, is the answer.
At the least, this means Chicago will play in 2007 with little depth at defensive tackle, while going up against the AFC West which features three teams, San Diego, Kansas City, and Denver, that are among the best at running the ball.