Mike McCarthy has called it a knee sprain and expects he'll be back for the playoffs. It's also been called an MCL tear by Mike Florio (yes, I understand the source is unreliable). My limited understanding of the injury tells me that a tear and a sprain are similar, but the difference is the severity. Either way, it doesn't sound too serious. From the Journal-Sentinel:
Late Sunday night a source said Jennings had torn the medial collateral ligament, but on Monday coach Mike McCarthy announced it was just a partial tear or sprain and that Jennings would definitely be back in time for the Packers' first playoff game the weekend of Jan. 14-15.
As far as personal goals, it means Jennings misses out on his 4th consecutive 1000 yard season (949).
As far as his absence on the field: I don't expect the Packers to miss a beat. Thinking back to 2008, the offense really seemed to be driven by Jennings. If he wasn't part of the offense, they were struggling. And that impression of them has carried with me.
For most of this season, he was recording a lot of receiving yards or catching a touchdown in every game. I had thought that maybe a key to slowing down the offense was to shut him down. But in their November game against Buccaneers, he was limited to only 2 catches and 6 yards. Instead, Aaron Rodgers threw two touchdown passes to Jordy Nelson, and Donald Driver had 4 catches for 72 yards.
While Nelson is sure to draw more attention, Driver has caught 4 passes in 3 of his previous 4 games, and had 2 touchdown receptions against the Giants. It also means there will be more opportunities for James Jones and Randall Cobb. Jennings is their best receiver and I don't want to ignore his absence, but I'm really not worried about them. He just needs to rest and rehab for a few weeks before the playoffs start, and then he'll be ready.