I was looking back at defensive coordinator Dom Capers press conference from last week. He was asked about the secondary and his response was "we're a totally different team." He might have answered that question in regard to the entire defense, but the question was specifically on the Green Bay Packers secondary. Why are they so different?
After CB Al Harris was lost for the season in November 2009, they were forced to call on CB Jarrett Bush as the nickel back. It wasn't a problem for the first few games, but when they met the Steelers, who have three big play wide receivers (Hines Ward, Santonio Holmes, and Mike Wallace), it was a disaster. He gave up a 60 yard touchdown on the Steelers first play of the game.
What's different is that nickel CB Sam Shields isn't Jarrett Bush. If you watch that painful highlight video linked to above, you won't see Shields giving up a few steps (I'm assuming on a double move) before getting spun around like a top trying to find the ball. Bush still plays in the dime package, but that usually involves zone responsibilities in the middle of the field, and the big play potential isn't the same on those plays.