It's been reported by the Journal-Sentinel that the Green Bay Packers just lost both CB Al Harris and LB Aaron Kampman for the year with ACL tears.
There's no way to replace what either player can do for the defense. Harris is their top cover corner, and Kampman is still their best pass rusher. A pass defense that is struggling after giving up another 3 TD performance to an opposing QB just got a little worse. With the game coming up on Thanksgiving, there isn't much the Packers can do to replace either this week. Not that there are Pro Bowl LBs or CBs looking for jobs right now.
I'd expect CB Trevor Ford and LB Cyril Obiozor are added to the roster this week from the practice squad, after Harris and Kampman are placed on I.R. Both played extensively in preseason. They are undrafted free agents who were signed back in April. Ford is a poor cover corner, but good in run support. Obi is a project of linebacker coach Kevin Greene's and a former college DE making the transition to LB. He's an athletic player, but didn't manage to put much pressure on the QB. If there's another injury in the secondary, remember the Packers have already lost CB Will Blackmon during the season, then Ford might play. Obiozor could be added to the active roster, but would still be buried deep on the depth chart.
As far as replacing them on the field, LB Brad Jones has impressed the last two week and he'll start in Kampman's spot. LB Brady Poppinga is his backup, with LB Jeremy Thompson behind them. Jones showed some promising pass rush moves this preseason, but his strength right now is against the run and in pass coverage. CB Tramon Williams will now start, with CB Jarrett Bush as the nickel. Though they've run a lot of sets with 4 CBs lately, so CB Brandon Underwood got some playing time after Harris went out. None of those players can replace what Kampman and Harris can do. The team will just have to try their best going forward, and defensive coordinator Dom Capers is going to have to make some adjustments to compensate for their absences.