The Green Bay Packers have improved their defense, but they now face the heart of their schedule without one of their best players.
The Packers later confirmed the injury.
McCarthy said the injury is similar to the one Woodson suffered in the 2011 Super Bowl, but not as severe.
The news comes out of left field. There was no hint of Woodson's injury during Sunday's 30-20 win over the St. Louis Rams.
Woodson recorded five total tackles in the game and was on the field for 50 of the Rams' 60 snaps. According to The Associated Press, McCarthy and defensive coordinator Dom Capers said Woodson broke his collarbone while diving for a pass with 2:44 left, but he stayed in for the next play. Woodson sat out the Packers' last defensive series.
"I went to him in the locker room after the game, and he wasn't sure (about the injury)," Capers said, via The AP. "He definitely said it was when he dove to (try to) make the interception."
The 15-year NFL veteran's play has declined in recent seasons, but the Packers aren't likely to shelve him for all of 2012. After moving from cornerback to free safety, Woodson has posted 38 tackles and 1.5 sacks. He also has forced a fumble and tallied an interception and brings unmatched experience to an otherwise developing secondary.
Woodson's play against the run has been solid, but the Packers will struggle to replace his leadership on a defense that has improved from among the worst in the NFL last season to 14th through seven games in 2012. Woodson has been a central figure on the Packers since 2006.
If Woodson plays again this season, the six-week timetable would place his return at Week 14, but that would mark a quick recovery from an injury suffered twice in two years.
Follow Marc Sessler on Twitter @MarcSesslerNFL.