Favre was hurt early in Sunday's game when his hand struck one of his own offensive linemen after a pass. Favre completed the game, simply icing on the sidelines.
Tests today revealed the thumb was cracked.
Sherman said he asked Favre how this injury compares to his thumb injury in 1999, and Favre said there was no comparison - this time around is minor compared to the previous injury which affected his grip on the ball.
Also, Favre can play despite the cracked thumb and is not likely to worsen the injury, Sherman said.